"...they will have come with a very high price in sustaining any sort of cohesiveness in the party in time"

There's no problem with that: they were nearly all on-board Starmers pirate boat beforehand & there's but a few stragglers of no consequence left.

"...Labour is not just a party, but it's a movement and an ideology of which radicalism is or should be a major part...without those fundamental underpinnings, the party will start to disintegrate... "

Good grief John... that train left the station a while ago.

It certainly is not a "movement" any more and it's present ideology is craven managerial-capitalism. Some would say the "Labour" you are talking about was always a mirage...or mere wishful thinking but it's of no consequence now as even the pretence of such a party is long gone.