Maybe
@Charlaquin is right and people are thinking a little too rigidly here.
As I said before 6-8 only applies if the battles are medium/hard. If the battles are deadly you can go down to 3, and you'd probaby have at least one 'deadly battle' in any given day (as hard is not really hard). So realisitically, the way most people tend to prep things, it's more like 3-6. (More than that is if the players are working through something like an old school dungeon crawl.) At 3 combats Fighters can still keep up, provided they're not spending two of those combats waiting to get their action surge back. (I'm not so sure about Warlocks, I banned them in the first game I ran and I've yet to see one in play despite allowing them since and hence have never looked that closely at them.
So there's two issues that I have seen arise.
1. Games in which the number of combats during a long rest is reduced, but the frequency of short rests doesn't change.
2. Games in which the number of combats during a long rest is reduced to one or two.
There's also issues in how the game plays well which are more subjective. At about 3 combats, the PCs will be much more powerful, and combat will take longer to challenge them - this becomes a game much more about the set piece combats - which I don't think is a strength of 5e.
At less than 3, the game becomes extremely swingy, as the PCs throw everything they've got with abandon and the GM has to keep upping the ante until they accidently up it too much and accidentally TPK. Even at this level when playing in such a game my rogue did not actually feel 'weak' the party needed his single combat damage. It was more that combat took so long and clerics and sorcerers were pouring through their spell list and doing multiple things on their turns against multiple opponents while I was just too doing my boring predictable sneak attack every round.
The game is pretty resilient. As I said at the beginning it's a matter of 'better' or 'worse', not really of broken.
We then played one game with a different GM in which the party fought about 8 combats (at least two of them 'deadly' and 3 short rests with my Fighter ending the day after 3 action surges and with 4 hps and no hit dice, and it was obvious, after playing through the former, that the game was designed to work this way. Individual combat was faster - much faster - combat rounds were quicker because the spellcasters were using cantrips about half the time, the Fighter was clearly more important to the party; Action Surge swung a combat multiple times.