I think that's where the niche is. Anything that isn't available elsewhere at all or is only available in resin at higher price seems a sensible place for 3D printed models to aim. But like the OP I just can't see much sense in producing stuff that's already available in plastic at higher quality for a similar price. 3D printed 75 or 76mm Shermans seems silly, but something like Sherman ARVs? Game on.
To me paying someone else for 3D prints just isn't going to be economical anyway. Surely the advantage of the technology is being able to print them yourself? You avoid postage and the price per print is so low you offset the cost of the printer fairly soon. Paying for time on someone else's printer to produce a model covered in striations seems bonkers when you could just pay a similar amount and get a nice injection moulded or cast one.