I 'm reading some of these posts and

It is obvious MSO5000 has much more noise, and that it is a problem, unless you only look at digital signals and just want to look at general shape and some timing information.
It is shame, really, because new Rigol scopes are much more powerful processing wise than the old ones, and generally held great promise but analog front end/ADC noise performance is not very good.
Scope with low noise is always going to be better instrument than the one with high noise. Why is that even a discussion?
Is this some audiofool discussion how this huge noise is pleasant to look at because it's pretty?

I don't use bandwidth limiting, averaging or any "signal cleanup" features when I'm looking into a signal I want to understand. You would want to look at this switcher signal with a full 1GHz bandwidth and with as low noise scope you can.
To really see what is there... Switching ripple is most of the time least interesting part of switching PSU. We expect it to be there, and most of the time it will be roughly what we calculated. Other, higher frequency stuff (those little hairs on top) is much more problematic and most of the time those will give you headaches.. Nanovolts of those will already be seen on any EMI test...
You filter, limit and "cleanup" signal in circumstances where you understand your signal and you want to ignore noise and other parts of signal on purpose. If your signal is buried inside the noise, you average.
But is that noise part of signal you're measuring or your scope is not irrelevant. If it comes from DUT I want to know that. I want to see it..
Only way to do that is to have low noise scope.
Of course, like OP correctly asked, there is a point of diminishing returns..
Is scope with 5 uV of RMS noise so much better than one with 50uV RMS noise for measuring this switcher signal from this example? Probably not.
It would be definitely better but probably not usefully so in this case. But one with 50uV of RMS noise is definitely better than one which has trace that is whooping 20 mV wide... On a signal that is 60mV P-P...
On this test I would call MSO5000 from Rigol useless for this measurement. And averaging this
not autocorrelated signal ( it doesn't repeat cleanly and doesn't retrace it's waveform exactly but varies slightly all the time) will
not extract more detail but will hide even more information about signal..
OTOH Siglent shows pretty much perfect representation of the signal, big peaks, ripple AND little hairs. That is your switcher output. That is
useful information..
Little Micsig STO1104C/E, or Siglent SDS1104X-E could do equally good job here.
Sad part is that little Rigol DS1054Z would be much better for this signal than MSO5000.. DS2000A had excellent low noise front end .. But new series of Rigol scopes is very powerful in processing power but analog performance is worse than older series. Shame really, otherwise they are very nice scopes.