I imagine most 20s will still stick around but there will probably be a lot more /afk at the start of games. I'd be more concerned about faction balance and how it'll affect queue times.
F2Ps have always been the majority of players in the 20-24 bracket. The bracket itself became popular after certain OP classes received nerfs in Cata and less powerful classes received buffs. To put it simply, specs and classes which have been underplayed in recent years have seen a boom in Cata and MoP, and now make up the majority of the classes played @ level 20. The addition of BG scaling also helped this happen by giving level 20s the ability to compete against level 24s.
Currently, there are only a few classes/specs which have a distinct advantage by going to level 24:
- Spriests (gain Power Word: Fortitude, Mind Blast, Shadow Orbs, and Devouring Plague)
- WW Monks (gain Fortifying Brew, Touch of Karma, and Legacy of the Emperor)
- Any Spec Mage (gains Ice Lance, Inferno Blast, Arcane Missiles, and Fingers of Frost)
- Any Spec Warrior (gains Pummel)
Everyone else either gains nothing between level 20-24 or only non game changing abilities. The gains of Spriests and Mages coincide with the fact you don't see very many people playing those classes/specs @ level 20. They literally don't get their crucial abilities until they level past 20. You don't see F2P monks (for obvious reasons), but you do see a fair amount of warriors @ 20. Pummel tends to be less important to Prot warriors because they can just hit you for 800 shield slams.
In 5.4, with the down scaling and sudden availability of end game enchants, you will see every class become more powerful just by being level 24 (or P2P). Rogues/monks/shamans/druids gaining the ability to use Mongoose will set them apart from their level 20 counterparts. Elemental Force will be a very viable alternative to mongoose for players who want more front end damage. Warriors and paladins won't see a huge increase in power @ 24 because they can already use Crusader, which will still provide the highest benefit when compared to higher level enchants.
Hunters will gain access to high level scopes @ 24, X-Ray scope giving 150 AP on proc. Hunters are also getting each of their main abilities buffed.
Casters and healers will be getting intellect/spell power proc enchantments, giving them an occasional boost to their primary throughput source.
All classes will be getting access to leg armor kits and shoulder enchants. Players who applied helm enchants to BoAs prior to MoP will have those as well. Secondary stat enchantments (haste, crit, expertise) will also become available to P2P players, giving them the ability to further customize how their toons perform.
All told, anyone playing a level 20-24 P2P character will see a 15-25% power level increase just because of 5.4 changes. Some classes and specs will benefit more than others. The bottom line is the disparity between P2Ps and F2Ps will be greater than ever, and there will never have been a better time to roll a P2P in this bracket.
If F2Ps stick around, it's because you either don't know any better, or you are some sort of a masochist. I don't foresee there being much competition between F2Ps and P2Ps, in 5.4.
