There are certainly applications where a long record length is required but they are in the minority.
Not really. For example, just watch how quickly the sample rate (and thereby the useable BW) drops on scopes with small sample memories when you extend the timebase. A scope with deep memory can sustain a high sample rate even at long timebase settings.
But not long delay settings as we discovered with the Rigol DS1000Z. (1) In that case, long record lengths are great as long as what you want to see what lies within them and if it does not, the sample rate has to be decreases anyway. Oscilloscopes with short acquisition memories use features like peak detection and delayed acquisition (sweep) to apply their maximum sample rate exactly where the user wants.
More processing power was also required to allow deep acquisition memories but both were the result of increases integration and processing power has fallen behind making very deep acquisition memories *less* useful in a general sense. Maybe high end DSOs avoid this problem but my experience with the DSO/MDSO5000 series is that they do not; using high record lengths results in waiting for processing of each record which is fine for single shot applications where long record lengths are especially useful but it is aggravatingly slow otherwise.
This processing power problem with long record lengths is not new. The ancient Tektronix 2230/2232 DSOs support 1k and 4k record lengths which seems laughably short by today's standards but why did they support a 1k record length at all? Why wasn't more acquisition memory included? It would have been trivial to do and only moderately expensive. I suspect it was because the limited processing power available at the time could handle 1k records significantly faster so for a *better user experience*, a selectable 1k record length was made available.
(1) The Rigol DS1000Z series brings up another question. Exactly what is the record length of a DS1000Z? Measurements are only made upon the display record which is 600 or 1200 points long yet the specifications say 3 Mpoints/channel. Shouldn't they say something like 600 or 1200 points operating in real time and 3 Mpoints/channel when stopped? How many other DSOs which make measurements on the display record are like this?
If only 600 or 1200 points are displayed, what happened to the others? What exactly is the sample rate of a Rigol DS1000Z when all of the samples do not fit into the display record? Do they all contribute something to the display or are they thrown out? This may explain some odd looking displays on modern DSOs.