Two equally skilled fighters meet up on the battlefield, what happens? Yes the one with the better equipment and stats most probably wins.
Crossbow + steel bolts = 3 slots ---> only one 1 slot is left. Crossbow and steel bolts also tend to weight a lot (not like arrows and a bow but it's still a lot). The player with the 2hand/polearm with practically better armor or faster movement will have an advantage over the crossbowman in melee fight.
Yes the crossbowman can still win, and personally speaking I mostly do. That's because, however I mostly face with players that are worse than me in melee (I think I'm above avarage in melee). So I somehow counter that effect. If I face with someone with an equal skill or worse someone better, then I am the one having a harder time mostly. In high level duels, those differences make up the whole difference! If you can't use your advantage against a hybrid, you're already a worst fighter than him!
Raising the STR requirement does not sound like an extremely effective idea. One can always do well with a less powerful crossbow if he can't use... say... arbalest, for example. Personally speaking, I use a crossbow with 21/21 build. I feel pretty effective, so unless you make even the lightest crossbow 30 STR requirement or something this won't really change all that much. 15 STR is a bit of a joke, but 18 STR won't really cut it off too. It will only make a few players angry when they can't use their favourite crossbows with their current build. It is only a matter of time before they get used to another crossbow.
Before someone criticizes me for being a lobbyist, I want to say... well ... whatever. Those people are idiots already.