Smart trainer bike

It's possible that there's an opportunity there, but it's also a very niche market: people who want to use a direct-drive trainer but with a bike attached to it permanently. As opposed to either a wheel-on trainer or going all out and getting a Kickr Bike or equivalent.

Then there's the economics: I doubt the cost of the material that goes into a cheap Alu or chromoly frame is very high; most of the money probably goes into the cost of manufacuring, buying the components (you can skip the brakes, but even so a 105 or higher groupset will set you back), and assembly, all of which will still be required. Given that you can already buy a new 105-equipped bike for <£800 (Triban RC520) plus the second hand market, and it's a small market to begin with, I can imagine it would be tricky to balance the books.

I do know people who have bought or made up trainer specific bikes; usually a v cheap frame, single ring chainset, and a chain. If you have a direct drive smart trainer, you don't even need gears or brakes. Add bars, seatpost and saddle. You're still looking at a few hundred quid, though. Personally, my winter bike goes on the trainer, unless I'm trying to dial in something positional on my good bike, or testing a saddle (trainers are good for that, as it's such an unforgiving situation if a saddle is uncomfortable).