That would be the most straightforward for a player to understand. But the current implementation i think is easier to use.
When T=0 but C=1 the inserters and pumps aren't active anymore. This is desirable because if you make your train leaves when iron >75 you expect the train to leave with less than 2 stacks of iron, not a quantity that depends on how many times it took for that train to find a destination.
that's not the best example of functionnal use i admit it's the first visible difference i had in mind when considering changing the current implementation so that T stays at 1.
The other change would not be to make T stays positive until the train actually leaves, but to remove C when the train has fulfilled its condition. And in this case the problem is that you can have 2 trains at the same stop while the limit is 1: the train that hasn't found a destination and never left, and the new train that arrived because C was removed due to the previous train fulfilling its condition, ( in this case it would break the no signal possibilities ).
Considering the 2 the least troublesome would be to keep T=1 as long as the train hasn't actually found a next destination. But in this case it wouldn't prevent the soft deadlock. (it wouldn't be enough to change 'just' this to solve the problem it would also require the 'pairing' algorithm).