@botlord and anyone else interested, just did the math. Assuming level 7 scientists, and ignoring the fact they get a few fewer multipliers than tile based resources:
1. The first 4 colonies (total 5 planets) were worth it, if you had art monuments, even if you ran out of scientists to assist research.
2. No additional planets are worth it, if you've run out of scientists. So, having more planets than scientists, above 5, is explicitly bad.
3. Without art monuments, colony 3 (planet 4) could be worth it, or not, depending on how many systems you need to claim to get to it, without a scientist to assist research. Colony 4 is almost certainly not worth it. So, never have more planets than scientists.
4. With a large number of colonies, depending on balance of habitats to conventional planets, your overall tradition gain speed can drop by maximum 25% (actual max depends on things like ethics and habitable planet density, for most empires it's less) or so by building many many habitats vs having many many planets (or ringworlds). This is true regardless of art monuments or assist research; having those just makes the drop more precipitous.
So, as you can see, assist research never encouraged "expansion for empires playing tall" without a complete misunderstanding of what tall means. An empire with a lot of territory, and a lot of planets, is wide. An empire without a lot of of territory isn't wide, but it's only tall if the planets in that territory are each better developed (which is mostly only true for a short while as they'll catch up in minerals and pop growth, or a little longer if you're ahead in tech which your post implied you shouldn't depend on) or if you've built up your territory by building habitats and ringworlds (which the old faith in science actively discouraged.) If your planets aren't stronger, and you haven't built more planets, you're not wide, but you're not tall either. You're small and short.
So, the old faith in science wasn't good for
tall play. It was good for small, short play, by providing an almost completely tradeoff-free ability to make a few planets appear far taller in the specific field of unity production. The new one is better than the old for wide play, and even better for tall play, since actual tall play (with tech and habitats) unlocks techs faster. The goal is for tall or wide to both be viable, nobody said anything about small and short.