In 5E, that trap doesn't spring by level 20, and WotC has set 20 as a hard limit to the game, probably for this very reason (beyond people not playing much into third Tier levels
The 'trap' is sprung earlier than the whole campaign starts to suffer from its effects, which'd be after you leave the sweet spot which reaches 11th, inclusive.
But, without some vigilance and compensation by a good DM, it'll start to wear on the trapped player long before.
But, then, 5e worked some minor miracles in Empowering good DMs for a reason.
There is a more subtle change in 5e that helps a little with the LFQW issue: the number of spell slots.
True. Like someone, maybe [MENTION=5142]Aldarc[/MENTION], said, 5e 'tweaked the curve,' yeah.
LFQW returned with a vengeance in 5e, but was only back, not badder than ever like in 3e.
But in 5e, things are different. You start out with more slots at lower levels...<but top out with fewer>
Some classes (wizards and land druids) get a feature to recover some slots on a short rest once per day...
Of course, neo-Vancian casting combined with cantrips, means that you can probably use your slots more efficiently, but it's still a factor to consider.
Yep, 5e casters get more spells at low level, plus attack cantrips, but fewer high level spells, and fewer slots, overall.
They also don't blow through near as many: you don't need to stack buffs to keep up with the fighter or precast whole slates of spells for major battles. A spell,especially a concentration spell can often turn a whole encounter.
And, everyone casts spontaneously, there's no concentration to cast or loss of spells from interruption, so slots are rarely ever wasted nor go unused because you memorized something you didn't need.
None of which bears directly on LFQW, which is about progression.
In the classic game, LFQW was arguably part of a balance scheme. Fighters (and multiclass non/demi- hunans) were profoundly superior at 1st, but fell to virtual irrelevance at higher levels due to LFQW (and racial level limits).
By 3e, Mcing was the same for everyone, and casters were competitive at first level, but restrictions on casters vanished and LFQW was only slightly tweaked by damage caps and save DCs scaling with slot level.
4e, MCing is barely a thing, Races & Classes are reasonably balanced, racial level limits & LFQW are gone, and feats are a few must-haves in an insufferable cloud of chaff. Casters are on equal footing from the start (and the rules are simpler & easier on casters than ever, with AOs, but no loss for interruption only on range/area, and only wizards prepping & simply /one of two spells per slot, and at-will attack spells).
5e only made it even easier on casters, no OAs, no spell loss, all spontaneous, multiple at will attack cantrips. At first level, casters are doing just fine, even before casting one of their spells &/or after being tapped out. But it added back LFQW - even if slightly tweaked in opposite directions as 3e, by spells scaling with slot, but save DCs with level - that's an issue.
And, compared to the prior ed, in no way a fix or improvement.