Some thoughts:
1. For every "top quality rebuild," there are probably ten "low quality rebuild" that wouldn't match up to new pianos using a comparable scale design. It's the skill of the rebuilder that's the primary determinant.
2. Do you know if the rebuilder has modified the scale of the your piano during the rebuilding process, perhaps using modern scale design theories not available in 1915? Used a modern set of action rather than refurbishing the old ones? Used hammers made out of wool processed with modern techniques? Put in new soundboard and pin block manufactured using modern techniques? Used modern glue and finishing material not available in 1915? Employed modern piano building tools and techniques not available to 1915 craftsmen? In other words: how much of your piano is original all the way back to 1915, how much of it is modern?
3. And then there's a matter of personal preference for sound and touch.
Look up Steinway and Sons' patent portfolio since 1915, it's available on their website, and there are MANY patents. Other piano manufacturers must have their patents as well. Granted some piano patents are aimed to lower manufacturing cost. Still, have all those patents/inventions really not improved the "state of the art" at all since 1915?

That said, I agree with you that the planet is running out of certain types of wood traditionally/conventionally considered "best" for sound boards.
In any case, you are lucky indeed to be in possession of an instrument that you hold in such high regard.
