TynnyriIn Highlander, every class has restriction of 1. With 9 players, this forces 1 of each class. However, highlander never seems to devolve into a stalemate where neither team is attacking or defending. The "flow" of a match is pretty similar to a casual match in this regard; Both teams slowly poke at each other, then explosive high-value picks (Heavy, Demo, Medic) ensue, the winning team pushes the advantage and the teamfight is quickly over. (Of course it is more nuanced than this)
you clearly have not played long enough to remember the medieval torture of playing 5cp highlander on gullywash, process, snakewater. I would bet that more than 2/3 of games that weren't a roll on these maps ended in the 30 minute halftime timer running out twice. if we played 5cp it would absolutely be stalematey again. the reason modern HL doesn't devolve into stalemates is because we don't play maps that have average 30 minutes halves anymore - KoTH has a maximum of about 8 minutes per round, payload 30 if both teams are just really bad at offense but averaging 20 including setup time.
3/5 highlander classes are defensive in nature (pyro, heavy, engie) and while the other 2 are pick classes, generally spy in 5cp is most valuable in doing nothing but communicating with the team which choke point the other team is planning on pushing so that you can continue stalemating them.
meanwhile every 6s class is supposed to be fast-moving and way more viable at busting through chokepoints and flanking. while stalemating has been a problem in 6s, it would be far far worse if full time HL classes were mandated as well.