. you think the tree has to be filled
No, I think that the branch of the tree your champion is specialized in should be able to be mostly filled at mid-level, and after that the champion doesn't really need to progress further, although it could occasionally be nice to have a very powerful champion with a more or less completely filled trait tree even if it comes at the expense of the development of other champions. Instead, we have the ability to mostly fill the specialization branch of one of our champions at the expense of the development of our other champions, or just begin specializing multiple champions.
It's fine if it takes a lot of effort to completely fill the trait tree, but champions more or less stop developing around level 10 or 12, depending on how many you are trying to develop and how much experience is available, and at that point they've barely touched their specializations. Making things worse, the specialized branches of the trait trees tend to be deep and narrow, meaning that all the champions who specialize in something similar end up having mostly identical traits, because there are either no branches to take, or the branches are very short, or the branches only start high up in the tree. It's even worse for your champions if you take anything from the general tree, because the Potential traits don't cover themselves until you'd normally be level 16 or higher and the other traits, while useful for your champion, don't really add anything special or particularly useful (not including developing magic abilities).
At level 10, if you've completely focused a champion on developing one branch of their class tree, you have filled about half to two-thirds of that branch. This makes it feel like the sort of trade-off a low-level character makes - you can drop all of your generalist abilities to get halfway through a specialization, or you can have your general abilities and barely scratch the surface of the specialization - rather than the sort of trade-off a mid-level character makes, where you can drop all of your generalist abilities to more or less completely filling out a specialization, or you can have generalist abilities and get about half-way through your specialization.
this may work for other games with more linear trees, but it's quite obvious to me that this isn't the case for this game
How much more linear are the trait trees in those games you're playing? Each of the classes has two, or sometimes three, specialization branches, which either barely branch until you've invested half the levels you're going to have into the specialization, or which have a handful of stubs that don't go anywhere. For that matter, to some degree I'd prefer a lot of short (say, five or six traits), very linear branches to what we currently have - branches 8 traits deep with four or five additional traits strung off of them in random spots, which mostly require the same set of traits to be taken to get to in the first place - because at least then I could start to mix and match pieces of them. Right now, unless I take Tactician at sovereign creation, all my field commanders get Leadership I-III before they can really start to differentiate (and Trainer isn't really that appealing of an alternative to moving through Leadership IV to Tactician, anyways). Governor commanders have a nice variety of choices available, but I don't really want a legendary governor - I want a legendary hero. Damage and curse mages can mix up traits reasonably well, but summoner mages are more or less stuck with one track and the choice between skeletons or high-end summons. Assassins look like they have a nicely branched tree, but dodge bonuses are only really useful early on, and unless I get a good weapon in loot (or pay the ridiculous sums of gold for something from the shop) critical hits are only really keeping the assassin at best on par with the damage dealt by regular troops, and the assassin is generally more fragile than regular troops anyways, and while Executioner might be nice, champions aren't hard enough to kill to make it very useful. Warriors have two very linear lines of development and a bunch of weapon specializations, but weapon specializations are very loot-dependent in utility, the utility of the counterattack bonus is dependent on what you're facing, and the bonus for Mighty Blow is dependent on being able to make use of that ability without over-exposing your warrior to the opposing army, which can depend greatly on the random initial positioning you're stuck with at the start of a battle. And as for Defenders, magic isn't generally a significant enough threat from the AI and monsters to make the spell resistance branch worthwhile (and the spell resistance line also doesn't grant access to plate armor), while the Healer branch is tiny and the Defender branch has only tiny side-branches (which are often only marginally useful).