TF2's original design only includes slow HoT's (mediguns, dispensers) that force you to position somewhat close to one another as a downside, with the only exception being health packs which force you to wait for them to recharge and position in very exact spots (which usually favor particular teams or holding particular areas depending on the pack). This was actively chosen because all of the damage classes (except Pyro coz Pyro lul) did extremely high burst damage at their respective ranges, the intention being that stalemates would rarely happen because the damage in the game would generally always outpace the healing to an absurd degree, if you land good damage you could push on good damage.
Burst healing on the level of the crossbow has skewed the game pretty hard, because the game's damage numbers weren't designed around players being able to receive burst heals on a tiny reload timer with an infinite LOS requirement.
Take something like Gully's flank to 2nd, if you ate a good rocket back in the day you were boned, you hopefully had enough health after all the pressure to jump to your Medic otherwise you either walked up slowly (ancient Gully) or equalized up and then you waited for your 24 HP/s while your Scout was alone on flank, making your flank vulnerable and very easy to overrun.
Nowadays if you eat a good rocket you back up a couple steps and get crossbowed and the entire flank momentum basically gets reset. You need to sync damage before the xbow hits to land a kill or else you're not making much headway with the Medic up.
Plus it's basically a Scout buff, Medics are able to heal Scouts so much more often after they buffed the Crossbow's reload time + passive reload attribute because they can crossbow everyone else and spend less time healing everyone else directly and invest that time into Scout. If you tried to heal Scout as much in a no Crossbow world, you'd be forfeiting heals directly on your Soldiers/Demo.
Mind I just think they need to make it work off the crit heals mechanic and dump the passive reload stat, Medic's healing power is basically tied directly into crit heals and the crossbow completely ignores them which is pretty dumb. Players aren't supposed to receive good healing in combat unless they grab a pack, and Crossbow is pretty much ghetto packs in its current state.