I would not agree. They will feel strapped for resources alongside long rest casters without their short rests, particularly on one encounter days, and especially if those encounters go on to ten rounds or more.
Let's say I have a 7th levell full spellcaster. They have 4 first level spells, 3 2nd level spells, 3 3rd level spells and 1 4th level spells - and usually they have additional abilities beyond that that require actions. That is 11 spells plus additional things.
In a 2 combat day, some of them will get to use a feature like Arcane recovery to be able to cast two 4th level spells. That matches the spell casting of the Warlock - assuming the warlock has not items that offer an additional slot (like Pearl of Power, Rod of Pact Keeper, etc...) and can't short rest between combats.
That means we're balancing the 3rd, 2nd and 1st level spell slots (plus additional abilities) against the Warlock other abilities like Invocations, Pact Benefits, Class Features, etc...
As an initial point, how many of those remaining 10 slots can meaningfully be used in those 2 combats? 4 or 5? Maybe 6? You just don't have time to meaningfully cast all of those slots within 1 or 2 combats.
So that means we're looking about half of those slots (plus calss abilities) versus the Warlock's remaining abilities. And if some of those used spell slots are 1st level spells, let's remember that Eldritch Blast is often outperforming a 1st level slot once you hit 5th level. Would you rather do an automatic 3d4+3 (10.5) or rolling for 2d10+8 split between two attacks?
So, really - at this point we're balancing a few 2nd and 3rd level spells against 2 or 3 of the invocations of a warlock. And when you do that, it is pretty clear we're on pretty even terms.
Honestly - having played a few Clerics, Bards, Druids, Wizards, Sorcerers and Warlocks in 5E: They're all just fine. Yes, you 'feel' the restricted slots of the Warlock - but that is the point of a restriction. You're supposed to feel the constraint. If you didn't feel constrained, it would be a problem.
If you want a 'fix', my suggestion is to grab a couple levels of sorcerer, or grab the Shadow/Fey feats that grant a couple spells.