"Prestigers" before this question gets asked again.
I would wait before discarding your prestigers. Especially the more powerful (evolved forms). I think the motivation meter will be affected by the CP of the attacker. Photo below shows a 2400Cp machamp taking down a 1300CP ursaring. He only loses about 200 motivation. But what if a 1400CP umbreon takes down a 2600CP espeon? How much motivation could be lost? Don't know, answers come when you wait
...
Edit: that is another reason why i think that the chansey fascination has come back. Low CP w/high stamina & decent defense

Answers
Couldn't agree with you more. I hope it is not the case. This update sounds well to good to be ruined by something like this... Also, we have no idea how did Ursaring's motivation meter looked before the fight, but if this 200ish motivation lost is around half of the cap then I think we have nothing to worry about...
Except if the spoofer problem isn't fixed they won't be much of an option for many players. Add to that the fact you can control the timing of your gym battles (something I used to greatly enjoy) while now to actually fight an opposing team you have to wait until there is a boss battle instead of just taking down an opponent gym? Lol what
Add to that I have no real interest in playing Pokemath-wars with opposing teams. I just want to decimate their defenders as efficiently and brutally as possible. Kind of the point of battling.
Posted this in the Chansey thread:
Will be interesting to see if that plays into anything. I honestly hope not or it just ends up being a big time waste. I'd rather have gym battles be challenging and something you need to figure out rather than just beating a low/crappy CP pokemon 7-8 times just to knock off 200 motivation each time.
That can't be what the end goal is for Niantic and this "new" revamped gym scene. Would be such a let down.
The response would be, if something is 800CP, -200CP for fight lost, would equal 4 fights. If a mon is 2800 CP, -800CP for fights lost, then it still takes 4 fights (plus more portion usage). The higher CP still is better, but the system in question makes it a more equal playing field for all trainer levels... though some will take advantage with a "high level" Chansey.
The more I think about this the more it has to be a set/predetermined value. Otherwise as the poke gets weaker (less motivated) it actually then becomes harder to reduce motivation - I.e, takes longer as less motivation will be knocked off at lower CP? Can't be right.
So lets say a Slowbro starts at 2200CP. It would be easier to knock off motivation @ 2200CP than it would be at say 800CP. It should technically be easier to beat at 800CP but you would knock off less motivation than when it was full at 2200CP.
I'm not sure yet....
I have a massive (yes massive) prestige team so I'm honestly not worried about this. Having said that tho, bringing a 3000+CP poke to battle 1200 CP chanseys will still be a joke and require less healing. You will plow through crappy 1000CP mons with 1 Tyranitar and barely need any potions.
Just wastes time. So you may knock off more motivation with lower CP but it will take longer and use more potions. Could still be faster plowing through gyms with a monster attack team.
A) I doubt very much that the amount of motivation lost will be on a sliding scale based on the attackers CP. I could be wrong about this, because anything can be coded, but this would be 1M lines of spaghetti code to calculate all the different variations
b) I also doubt that it is a percentage of current CP. Reduce any number by half, then do it again and again, it will never hit zero.
C) The easiest way is to take a set prcentage of the INITIAL CP and subtract that set amount every time the mon is defeated.
D) Don't forget the loss of motivation if the mon isn't fed berries. What is the loss amount here? How much CP is restored per berry fed? A Golden Raspbeery increses a mons motivation to maximum, but how much does a single regular berry effect it?
My big question that I haven't found an answer to:
If there are 6 mon in a gym, and one of them leaves the gym either by defeat or by not being fed, or combination: That gym now has 5 mon. Can another be inserteed, or does the gym stay at 5? You cannot prestige it, so what happens to the 6th slot? Could a mon be inserted even if the gym is under attack?
Looking at the OP for this thread: I disagree with a statement the OP made. The Ursaring doesn't "lose about 200 motivation". It appears that he loses 1/2 of the motivation, (nowhere is motivation given a numeric value) and that causes a loss of 293 CP.
The descriptions of the new system state that a loss of motivation causes a temporary decrease in CP. Also, that a mon stays in the gym until the first battle loss AFTER it's motivation gets to zero.
Here's my assumption: A full 6 mon gym with all of the mon at full motivation/full CP
The first round, an attacker defeats all 6 mon at full strength. Each mon loses 1/2 of it's motivation, and thereby loses some amount of CP. The amount per mon will vary based on the initial CP of the mon. The higher the CP of the mon, the more CP will be lost. The attacker then starts round 2 and runs through all 6 again, and does so even easier since they all have lower CP than the first round. This further decreases their motivation by the remaining 1/2, (to zero) and reduces their CP again. Each now has zero Motivation, but none have left the gym because they haven't been defeated during a period of zero motivation.
The attacker then runs through them all one last time. This time each of them is kicked from the gym by the defeat with zero motivation. The gym is now empty and ready to be taken over. In all, the attacker had to defeat 18 mon. 6 per round, and each round should have been a little easier than the previous.
This also still allows for lower level players to take a gym. They only attack the first 3 mon for 3 rounds each, removing those 3 mon after the thrid round. Then the attacker can go after the second 3 mon the same way. The attacker still has to defeat each defender 3 times, but can do so in stages.