True, but there are other factors, like free rules for Chapter Tactics, Relics and
WL traits that also help decide what armies are great and most importantly there is points.
There is a significant amount of time between codex updates for some factions. Being able to use each stratagem once per game should reduce codex creep’s effects, allowing older armies to compete while recently updated armies can still use the shiny new strats each game.
Quite the opposite actually, the newer codexes generally have watered-down Stratagems but they have a lot of them, you will only make newer codexes better with this rule.
My Background:
I am not a competitive gamer. I like to play different lists and rotate in a bunch of unit types. I like the idea of stratagems, but I do not find it fun to get obliterated by Havoks for three turns of Veterans of the long war (1cp): +1 to wound for a unit, combined with Endless cacophony(2cp): (Unit can shoot a second time). These guys were the run-away all-stars of our group’s campaign and this stratagem combo has been responsible for plenty of tabling.
3cp every turn buys you: 64x Str 5 ap-1 shots, wounding up to toughness 9 on a 4+.
Usually accompanied by a buffing character for re-roll 1s to-hit. No minus to hit for moving on from reserves because havok’s special rule. Extremely good value for CP use, and I don't fault my opponent for using it.
Chaos Space Marines do not have their 9th codex, so what you are looking for isn't actually to bring up old codexes to new standards but to hurt two combo stratagems of an older codex.
I do think Stratagem combos are a problem, if shooting with Havocs is worth 2CP then shooting with Havocs with +1 to wound is worth more than 2CP, therefore you should not be able to apply Veterans of the Long War an extra time for free, these abilities should only last until your next attack this phase is done, not until the end of the phase. But it was not implemented with the new 9th edition codexes, so without redoing the whole Stratagem system it's too late for
GW now.
I think there are way too many rules for armies and Stratagems, I would like to see all faction Stratagems, Chapter Tactics and Combat Doctrine-esque abilities removed from the game in favour more universal Stratagems. To replace the flavour of Chapter Tactics each army list gets to pick 5 of those universal Stratagems, the rest become unavailable. An Imperial Fists 1st Company list should not necessarily have the same Stratagems as an Imperial Fists parking lot list.
Then re-implement Specialist Detachments with flavourful Stratagems, relics and
WL traits like combat doctrines and Specialist Detachments resembling the new flavour armies from War Zone Charadon The Book of Rust as needed. There needs to be a reason for a specialist detachment to exist, it needs to let you play with your models in a different way, it should not be a way to make underperforming units perform, just lower the cost of those units. Specialist Detachments come with the in-built balancing mechanism of costing 1CP, implementing rules this way also makes them optional, making it easier to learn the game because you don't have to learn about what 7 plagues you can spread or which turn your super doctrine activates.
If for example, Drukhari Wych Cult units are amazing, but only in combination with Drukhari Haemonculus Covens units, then it could make sense to make a Wych Cult specialist detachment that makes mono-Wych Cult work without making the Wych+Haemonculus list overpowered. But if Wych Cults are just bad then a Specialist Detachment should not be the answer.