When one of the lead designers explicitly says that a core design assumption--that you'd be getting two to three short rests per day, which is what makes classes like Warlock and subclasses like Champion work in 5e--fails to be true specifically because players take too few short rests per long rest, I don't think it's that weird to say that the length of the short rests vs. long rests might be part of the problem.
And yes, I've crunched the numbers. Champions can keep up with BMs if they get enough rounds of attacking each day. You only see the numbers converge at about 7-8 encounters per day, with 6 the gap is debatable, anything less it's obvious (when looking at aggregate numbers for the day, of course). Warlock is in a more-or-less similar boat; I haven't crunched the numbers as thoroughly as I have with Champ vs BM, but my looser estimates corroborate the "6 (mostly-combat) encounters is good enough, 8 is pretty clearly balanced."
Most groups have fewer than 5 combats per day, and most groups have 1-2 short rests per day. This is negatively affecting the play-experience of 5e, enough that one of the designers explicitly spoke about the problem of short rest vs. long rest frequency. The amount of time taken by short resting three times per day is a full third of the time you'd spend on taking a long rest. It definitely doesn't help matters.
I get that some people like having short-rests come with some kind of "cost." The problem is that they already had the uphill battle of convincing casters and other non-short-rest-based characters to take them. That extra "cost" in time-investment has made the 5MWD problem worse, not better, which isn't a mark in 5e's favor.