What latest bells and whistles? There has been very little fundamentally new in DSOs in the past 15 years
Well, I didn't know there were 100GHz scopes using technologies like DBI 15 years ago already (100GHz scopes only exist for roughly 10 years, and those were sampling scopes). Because I see something like that as quite a fundamental achievement.
This aside, there were lots of other advances within the last 15 years which make getting a modern scope worthwhile, for example higher sample rates, larger sample memories. Or much faster processing, which is the key to enabling even more advanced analysis functionality and higher trigger rates. Larger screens with higher resolutions, Decoding capabilities for a ton of buses and communication standards and so on.
aside from companies like Rigol bringing feature sets previously only found on $5000+ scopes down to $500.
No, not really. Rigol makes perfectly fine entry level scopes, but none of them (not even their DS6000 Series) has any remarkable features, in fact, aside from the large sample memory (which for the low sample rate of these scopes is dirt cheap these days), feature-wise they're pretty basic and in some areas (like FFT) are put to shame even by many scopes much older than 15 years.
Rigol was one of the first manufacturers to produce decent entry level scopes at prices lower than the established big brands. That's how they got their slice of the market, by delivering decent quality at low prices. If they had to fight by features they would have starved.
Just look at the number of people who still use their 20+ years old 20-250MHz analog scopes today - some people have even posted restoration videos where they have to reprogram the flash/NVRAM/EEPROM memory due to bit rot. It does not matter how old scopes and other laboratory instruments are: as long as they still work properly and are still appropriate for some everyday purposes, people will find uses for them.
Yes, but so what? Of course as long as these old scopes work (or can be restored to work) they can be used for the same tasks they have been used 20 years ago. But that doesn't mean that they're also adequate for more complex tasks, like debugging modern buses.
For example: how much better will your scope from 2030 be at measuring ripple and noise on power supplies than today's DS1054Z or DS2012A? Likely not that much - scopes from 20+ years ago are still adequate for this today... some people even prefer analog scopes for this sort of stuff.
Well, the thing is that probably only a very small minority of EEs use scopes for looking at basic stuff like ripple and noise of power supplies, and if the PSU in question is a complex switch mode PSU then a modern scope offers much better analysis capabilities than any analog scope from 20 years ago.
In 2030 we'll certainly have even more highly complex electronics and buses than today, and all of them will need debugging at some point. Your DS1054z or DS2012A, which even today are merely at the lower end of the performance scale, will be utterly useless by then.
I much prefer having equipment that will last however long I have uses for it than being forced to replace it because it failed before I had any reason to buy something newer/better.
That's fine for you. Unfortunately not everyone looks at PSU ripples, and more often than not I find that some of the new features of a new scope saves me so much time or makes my work so much easier that replacing it pays practically for itself, even when the old scope still works perfectly fine.