It is absolutely adequate for photo editing, especially if you don't even do so frequently.
A computer is a tool, a fact a lot of people here seem to ignore and just encourages everyone to go for the latest and greatest regardless of actually needs.
A tool HAS to WORK FOR YOU. If your job requires you to have a better tool then upgrade to a better tool.
IMHO, if you are not making $$ with your photography and your 2015 MBP machine can accomplish your (seemingly very infrequent) photography needs, albeit a little slower, upgrading is just a want not a need, and only you can (or can not) justify the additional cost relative to what you hope to get back in return.