Do I have to worry about machamp being outclassed anytime soon?

Conkeldurr in Gen 4 will out-do Machamp based on its movesets.

But there's nothing wrong with powering up this one

I put Gen 3 instead of 4…

Let's see...
Blaziken: Hits hard, but like starters, somewhat short.
Breloom: Nice but frail.
Hariyama: Tank with HP, but not so hard.
Medichamp: Without special things of the main series, nothing to do.

So... Machamp at least until Terrakion in V Gen.

It is impossible to answer such question since the key determinant lies in the moves, not the Pokemon's stats.

The reason why Machamp is so good is that Counter + Dynamic Punch is one of the best move combos in the game, if not THE best.

There are many Pokemon who have access to Fighting moves, now and in the future. Take Mewtwo. A Focus Blast Mewtwo takes out Blissey faster than Machamp and has much better resistance to Blissey's moves. So, if a Pokemon with much better stats who can have access to Fighting moves, Machamp will be outclassed. It doesn't need to be Fighting Type.

This site rates those moves A and it has a 10/10 rating. Pretty darn safe to power one up***a month ago I powered 6 of them from level 20 to 2160-2200cp. Felt like it was me getting superjacked. Paid off a bit, I got three more Tyranitars via raids. One seems very reasonable

If you don't have a Machamp, that is a VERY safe investment. Do it.

I am personally considering powering up a third C/DP Machamp but probably won't do it since I'm doing OK on fighting mons at the moment. But I certainly wouldn't be without at least ONE very strong fighting type.

Even if it does get outclassed it's still going to be a strong fighting Pokemon, so don't regret powering it even if it turns out Gen 3 gets released a day after you finish powering it up.

I have no idea when they'll release Gen 3, but I'm guessing it'll be after legendary raid hype dies down, presumably when all the Gen 1 and 2 legendaries get released, so if you power up this Machamp now and it helps you get Tyranitars from raids, which will then help you get legendaries, then it's at least gotten some value from your investment.

Feel safe to invest in it. Nothing will outclass machamp in the next year and a half, at least...

It's probably a long way ahead. Just power him up.
Also, if you like Machamp, don't worry about him being outclassed, just power him up. I power up Flareon and Feraligatr, despite both being outclassed (the latter by a much more common Pokemon).