Perhaps this is a problem that will go away with time as newer devices are made available. Considering how many things are running on that one watch face, I really have to wonder if they've bumped the RAM on Series 4. Increased battery size and life (efficient chips) can also lead to more complicated faces that would otherwise drain the tiny battery in a few hours. If they provide an API where you can basically just skin the watch part and have a few zones for complications then that would be a good start. Then as older Apple Watches fall off in support for newer versions of watchOS they could keep adding things to the API to make it even more complex.
The other part that we both failed to consider is that Apple provides exclusive watch faces for lines like Hermes and Nike that could be copied. Though I doubt they would approve such copycat faces from third party developers, and could respond to take down notices if they do make it through. They may be worried about a flood of watch faces that copy other brands and since they're Apple they're more likely to be sued. That combined with everything you mentioned means they're just chilling for now.
I feel you are partially right here.
However, in addition I personally feel there is another reason.
Apple do like to keep things held back and in reserve.
We all know they won't allow any old watch face to be offered to users (as most want) as that would throw away any control they have and allow people to mimic big brand names.
They could of course vet them and release approved ones, and they will do this in time.
However for now, I'm 100% sure they need this watch to be a long run, very slow burn product, and they need to drip feed things onto it, year upon year, and hold back things they know people want, to gradually enhance the product in general.
Right now they are using the watch faces, and customization aspects as a marketing tool, trying to just throw on a new face and a different band and market it as something special, when in fact it's not at all, it's just a artificial special-ness.
This I'm sure they know only has a limited lifespan.
As I say they need to hold things back and drip them, if they give the users what they want on day one, then it's done and dusted and no more want will be there.
Give it time and things people want, and they full well know people want, will happen, but only when they feel they have no choice, perhaps there is little else left to do, so they will start bringing the old WANTS out to promote a far future version.