bike wrench

Ah, I see what you mean now.

I am not an expert at building or specifying whole bikes from the frame up -- though I have built wheels before -- but my understanding is that provided that the rear cassette, the chain, and the front sprocket are all compatible w.r.t. teeth width and spacing, and there is adequate clearance for both derailleurs from the frame and to the ground, I can't see why you'd be prevented from fitting a 2x5 or 2x7 setup into the space where a 3x7 used to be.

That said, when it comes to replacing non-like-for-like parts, you'll have to consider what your design priorities are. For example, would you want to keep the same "width" of ratios as you had with the old 3x7. That is to say, the final crank-to-wheel ratio in the new lowest and highest sprocket combinations would roughly match that of the existing sprockets.

I'm not entirely sure what style of bike a Rockhopper is, but the name suggests it might be geared like a mountain bike, so possibly lower ratios than a commuter might have. Then again, maybe you prefer a faster crank cadence and are fine with that ratio at higher speeds.

You should take time to evaluate what exactly you want from your drivetrain, and that will indicate what you would replace those components with. If you can find modern bikes with something similar to your wants, that would also work as a reference (eg parts, ratios, etc).

For example, I personally find that my 3x8 acoustic bike realistically could have been a 1x6, because I basically always leave it in the 2nd front sprocket and use only the even numbered rear sprockets, except for substantial grades that need the largest front sprocket.

Alternatively, for my Class 3 mid-drive ebike which is configured with a 1x9, I also only use every even sprocket, because the motor has a fixed cut-out time for every shift, so I might as well shift two sprockets at a time when accelerating from a stop.

I can practically feel the chain flexing

Supposing you were able to fit a 1x10 into your existing frame -- and it might not be possible unless you have a new rear wheel with a narrower hub -- the driveline could theoretically -- I'm guessing -- be optimized to prefer either the higher or lower ratios. In that way, you could minimize chain flex when in the most common ratio, although I wouldn't necessarily obsess about this too much unless you find proof that it's actually wearing away your chain or sprockets faster than expected.

Basically, sketch out a design, try it out, and let us know how it goes!