A few things to consider...

1. Bitcoin is not just software. It's a massive network of nodes, miners and people who currently hold their wealth and believe in it. That's incredibly difficult to copy.
2. Everything is a trade off. If someone figured out how to make a "better" Bitcoin what that really means is they are making different trade offs. This is the famous scalability vs security vs decentralization trilemma. Changing one impacts the others.
3. If there really is some undiscovered innovation that could be changed at the software level what stops it being implemented in Bitcoin?

The last thing I will say is that we don't know for sure what the market will value in the future. Is it possible that in 50 years from now governments stop printing money, become completely honest, transparent, trustworthy and friendly? In that world, Bitcoin wouldn't be as valuable.

It seems like real success would be whatever coin is adopted more widely.

Bitcoin is already successful. Is it adopted more widely than the US dollar? Obviously not. Adoption is only one metric to measure success. Real success is simply solving real problems and I would argue Bitcoin is already doing that quite well.