Not unfeasible in general, but as the default it is.
To be told yeah there are no coolers for your product that needs a cooler is a bit silly even for the largest company. There will have to be some easy to install cooler for these be they air or closed loop.
Custom is definitely an option, but it should never be the default. Imagine trying to sell the CPU and having to refer someone to a list of water cooling parts and then telling them they have to assemble a loop out said parts just to get their new CPU working. Custom loop is a luxury people undertake, not the starting point of cooling.
Some like me who is trying to build virtulation workstion and gameing machine all in one. I might pass on the x399 platform, it depends on how much more exspesive it would be to switch from a Ryzen 1700 to a threadripper.
Maybe there are no ideal options on the market right now, but Threadripper isn't out yet either and I am fairly certain that the market will offer you an AIO in time. The predator series from EK's line of AIOs has an easy to swap waterblock, so you could do that, if EK doesn't already offer a Threadripper variant when AMD's new chip is available.
Edit: For air coolers, Noctua already has some prototypes, that will probably fit.
Also this. I am interested to what a cooler for this looks like the CPU is huge. Plenty of options for intereting cooling, and in AIOs and custom blocks the larger area could make for interesting flow to adequately cool the entire chip.
Now my mind has raced and I am imagining custom blocks with 4 ports and two internal cooling loops. Half a chip each and heat transfer between the loops through the mid section of the block. Fun but totally impractical and silly. But still a fun idea.
I expect something like a new version of a D15 or something with revised contact plate will do the job.
It kind of wasn't. Nothing short of the largest Phanteks and Noctua's were effective all the time. the ended up shipping the 9590's with AIO water coolers.
EDIT: Though that is not to say you can't air cool Threadripper, new process, less heat, the the cores will balance that out. They should be fine, there is just currently no cooler, there will be.
With the way AMD is prepping this go-around, I really feel they're gonna give Intel a run for its money once Threadripper is released and if the rumor is true regarding their 8C/16T CPU costing $849 then Intel is really gonna be in for a rude awakening.