What's wrong with this? Rim brake on the seat stays. Pair it with a set of Hed Jet Blacks and I doubt you'll find superior braking performance anywhere.

What’s your budget? There are not a bunch of choices yet (P5X, Andean, PR6, Super Slice, Parlee, etc.). But if you like one of the choices, then go for it.

I am looking to buy a frameset only, no superbike (Price should be below 4K for the frame)

What frame-only disc TT options have you found? The only ones I can think of are Andean and P5X (I think). With those constraints, you maybe should wait until more framesets are available by themselves. However if you like the Andean, then go for it.

 
I suspect that within four to six months, you will have at least a dozen additional brands and models to choose from.

I'm pretty sure you can get the QR PR6 disc as a frame only, from places like MBS.

But I agree with the above poster that braking power is just fine with the correct traditional setup, and also, disc brakes can be a pain to work on too!

This. So much this. You don't really notice a slightly out of true rotor on a mountain bike or a gravel bike because you have quite a bit of noise from the trail/road but on a road bike on a smooth surface you'll hear the "tink..... tink.... tink...." of the rotor barely brushing against the pad and it is MADDENING. Kileykay had a lengthy post on this a few months ago.

Argon puts their rear rim brakes up high.

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Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
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Your question is a bit confusing.
If I read it correctly, the query is whether you should buy a new bike now or wait 1 or 2 seasons for disc brakes to become more mature before getting a new bike. Is that correct?

If that's your question, I would say you don't really need a new bike and it's strange you bothered asking the question. Wait and see what becomes available if or when you need a new bike.

If you want a faster bike, or a bike with better braking, surely you either want them soon or you're not really too bothered?
If you just want something new and shiny, use the possibility that you might jump too early to keep you in a holding pattern and avoid your frivolous consumerism. ;)

You outlined pretty well why disc brakes are more compelling for road bikes than for tri bikes, but you didn't say whether you have both a road bike and tri bike, or whether your tri bike is/will be your only bike. I recently bought a new road bike, and disc brakes were a must have. On my tri bike, however, braking performance is near the bottom of my list of priorities. I would never make a tri bike purchase decision based on whether a particular bike had disc or rim brakes. My current tri bike (P4) has mediocre braking performance at best, even by tri bike standards, and honestly it's never bothered me enough that I would consider it a reason to replace the bike.

I own a disc road bike, I am familiar with the benefits (and drawbacks). I simply feel the lack of proper braking is exposing me to risk I would not have with disc brakes (road or Tri).