Regarding R5 Resolution and Lenses
Resolution is stated as lpmm at a given contrast level, or contrast level at a given lpmm. Or even better, graphed as a MTF curve.
System resolution will always be weaker than the weakest component, but improving any component will improve the final result bringing it closer to that theoretical ceiling. Therefore it's incorrect to say that a lens out resolves a sensor, or a sensor out resolves a lens. Likewise it's incorrect to say you need certain lenses to realize a sensor's resolution, or that certain lenses aren't sharp enough to make a higher resolution sensor worth it. That's just not how it works.
Bob Atkins illustrated this in his review of the Canon 5Ds. He shot the worst Canon EF lens he could find on a 6D and a 5Ds, and the result was substantially sharper/more detailed on the 5Ds. People who argue about whether or not this RF lens or that RF lens is 'up to the task' of a 45mp sensor miss the point. A cheap consumer zoom from the 1990s will produce better images on an R5 than an R6.
"Equivalent megapixels" is also a grossly inaccurate and misleading concept. I get that even people who know better use this idea and terminology to try to illustrate to the masses what different lenses might mean, but it only misleads. It's one of the things I hate most about how DxO presents the results of their tests.
Megapixels is not a statement of resolution, but of sensor sampling rate which sets a ceiling on the lpmm axis of an MTF curve (Nyquist). So it's incorrect to say that a 150mp sensor with X lens would be equivalent to a 100mp sensor. There are no perfect lenses such that you could create a 100mp sensor reference point. Even if you did, that reference point would be valid for one specific 100mp sensor only as the sensor stack impacts the MTF curve. And even a lens that would be considered a poor choice for a 150mp sensor might still be able to resolve more lpmm at MTF10 than the sensor can resolve due to Nyquist. So what point on the MTF curve would we use to compare systems assuming a reference system everyone could agree on?
It's so much easier to just plot the MTF curve and understand it.
For stills at ±50mp we already vastly outstrip the possible resolution of most output apart from people with the need to use extreme crops and whilst there is a user base with that requirement it is not the norm or close to it. I am a generalist professional photographer and sell prints up to 24” x 36” and I use 20mp.
20mp 3:2 at 36" results in a print with roughly 152 ppi. (Going from R6 pixel dimensions as different cameras can be "20mp" yet be slightly different in file pixel dimensions.) That's acceptable, but a person with healthy vision can absolutely discern the difference between that and the 224 ppi of the R5 or the 241 ppi of the 5Ds/sR at close viewing distance.
To what degree it matters, if at all, depends on the subject, artistic intent, viewing audience, etc. But for some subject matter I consider the difference to be substantial, to be the difference between a good print of the subject and a truly immersive one. And 36" is not the largest print one can make or people can buy.
You certainly do start to get into diminishing returns, and we've been there this past decade with digital photography. But I wouldn't mind seeing 75-100mp 35mm sensors and 200-300mp MF (true 645) sensors.