First of all even if copyright was expired it doesn't mean access to source code.
Secondly, do you really think that having access to 20 year old version of Photoshop (source code or not) would be significant factor for anybody to enter the market? Making it work on modern systems would mean essentially a complete rewrite, at which point the original copyright would be irrelevant. Making it competitive would be probably more difficult with the original codebase than with a new clean design.