Mistaken gamentity
This occurred to me just recently, but it seems that Niantic has completely mistaken how games progress, and this is likely the reason why so many of the new Pokemon aren't that useful.
With the Gameboy/DS titles, you start a new game, and you have from hundreds of creatures to pick, as to which ones you will spend time training. They may have near equivalent stats in many cases, and in their case, speed and effects also matter.
Niantic believes that just keeping the same stats (or even nerfing them) when importing Pokemon into their game, that newer generations will be somehow more desirable to spend resources on than teams players have already built. Teams that are more effective in most cases.
The same methology can't be applied to PGO, because new Pokemon will be added every 10 months, whereas in the DS/GB games they're already within the game and available "at any time", so to speak.
In PGO, newer Pokemon HAVE TO offer something more, not just be a lesser-version of something already in existence, who wants that?
This also ties into the whole battle/gym/raid system, which if Niantic bothered to update at some point, could be more engaging and actually make different Pokemon with various effects useful.
Until then, reflect on one of the reasons why this game is stale Niantic.
Answers
If every new pokemon released is better than previous pokemon everyone would complain that all their previous work would be wasted...and so would not bother wasting resources on something they know will become inferior - and they would be right. This will have a detrimental effect on the game.
If no new pokemon make older ones obsolete by being stronger, then no one would continue to catch anything new (except for dex) as devoting resources to them would be called a waste.
Players would complain that new pokemon are nothing but dex fillers...and they would be right.
Niantic have to ensure their is a mix of both 'dex fillers' and mons that make older ones fall down the 'meta ranking'.
The funny thing is that even when they do give us a mix they get both of these complaints of ...
...nothing but dex filllers
OR
...make my investment obsolete.
We see these complaints for each generation or release of a new mon.
As you stated, Niantic have stats predetermined by the original game and so the one aspect they can use to progress is the use of movesets.
Funny enough, players complain that the current mons don't have the best possible movesets already...without taking into account that there needs to be room for some to be 'better' in future releases. If everything gets the best possible moveset immediately, then they will run into a trouble in the future where they can't give us something stronger or better.
It is a balancing act, that no matter which way it is done will lead to people complaining about specific individual pokemon in isolation because they can't see the bigger picture.
"would not bother wasting resources on something they know will become inferior "
Wrong. Though I will admit the battle system needs to be changed and an introduction of a PvP would help support this, this is already untrue.
Just look at any lv 3+ raid you have people maxing out certain pokemon because of a niche. You honestly don't think trainers would start maxing out pokemon that are best in their class?! Even if only by a bit, those bits start adding up.
Everyone wants the best most useful 'mon now. There will always be better pokemon around the corner, so the strategy is to wait for the ones you need best, and maybe skip over some depending on if you have an existing army of that type of pokemon and spending resources on where you think they'd best suit your needs.
If we actually went your way (hopefully not Niantic's..), then that means Machamp will be undisputed for a long time. All the other fighting pokemon don't matter. I think that's boring and uninspiring.
That's why I'm saying every generation of pokemon should be a little stronger in some areas, so it makes it more exciting combining new better pokemon and fitting them in with your current teams.
A species that is not objectively the best for their type can still have useful niches. For example, Hariyama has lower DPS but higher TDO than Machamp, so there are situations where Hariyama is preferable even though Machamp is overall better. What the game needs is more possible niches for such species.
As to "wasted investment", just introducing a better mon doesn't make the previous best in class mon unable to do whatever you invested in it to do. For example, if a better fighting mon comes out, Machamp can still duo ttar raids, and take down gyms. Other changes to the game do do this. For example, if ttar leaves the raid boss rotation, having a squad to take him down, is kind of a wasted investment.
You won't have lost anything. You'll still have used the pokemon for months or years by that point. Additionally, if you power-up current second-best pokemon of a particular type (ie Hariyama) to join your team, how is that any different later getting a better pokemon of that type and your current best then becomes second best, if only by a little bit amount.
I agree that it is a balancing act and that movesets are one of the few ways that Niantic can improve on the game.
I also think that the best way to make the game interesting again without changing PoGo from the original games is by including status effects like poisoned, paralyzed, etc.
This might be hard for the game creators, but it will open up a whole new gameplay for players. They can probably change some of the actual effects to something time based, ie, poison damage will be 1 damage every 2 seconds and can have counters that stack. Or something like paralyzed will increase the cooldown of moves by 0.5 seconds, etc.
Status effects might make many of the previously disregarded pokemon (mostly the poison types) relevant again as great alternatives, specially for new players.
