I'll add one more point which the OP missed, but which is also very important. If you're developing an alternative implementation, you probably have a different architecture from the canonical version, and things that are easy to do in the main implementation might be very difficult to do in yours.

Let's say there's a piece of proprietary software for creating financial reports[1], which is using a weird binary format to store their documents. You want to make a free alternative, which can load and save these documents. The format is not fun to deal with, so you have a single function that loads the whole document into memory and one that dumps your data structures back to disk and entirely overwrites the file, but you operate purely on in-memory data while the software is running. What you don't know is that the proprietary software doesn't do this, it was developed in the days when users had very little RAM, so it only loads and saves the section the user is currently operating on, and knows how to modify the document in-place.

Then, the proprietary software introduces a way to add attachments to their documents. People keep adding stupidly large files, recordings of investor calls, scanned PDFs with hundreds of pages and so on. THe proprietary software loads documents one section at a time, so this works just fine. You, on the other hand, always deserialize the entire document at once, which suddenly becomes a problem when the documents are bigger than the users' available RAM memory. You now basically have to re-architect the entirety of your software, to support a change that took a week of a single developer's time for the main implementation.

[1] a purely hypothetical example, I've never actually worked in this space.