Thanks for linking my post. I do feel this is the best way forward for Finale's longtime and loyal user base. Please keep making noise on this front and maybe in a year (or whenever the cross grade offer expires) MakeMusic might just decide to do it. 

Sadly, MM has told me via email that they have no interest in doing this right now. Reading between the lines (and all of their Dorico marketing) makes me certain MM is getting a cut of every Dorico crossgrade. It's one last sad attempt to milk the cash cow before killing it entirely rather than turning it over to a community who could care for and maintain the software until it reaches its natural end of life and volunteer interest and effort dry up. 

Plain and simple, Finale's source code will die because it's not as profitable to MM. That shows you exactly how much they love and care for the Finale user base and how much they respect Finale's place in the history of desktop music publishing. MM would spit on Finale's grave if it earned them an extra quarter. 

I signed the petition too but it is unlikely to do any good. If they did decide to open source it, there is very likely a fair amount of third party code mixed in with the Finale code that will have to be removed (and they may not have properly kept track of what is third party code and may need to get someone to audit the code, and pay them for months of work). The third party code may be fairly substantial and then major parts of the code may not make sense if this is removed. Even if you got really lucky and didn't have to remove very much code to clear out the third party code, it could still take a huge amount of time for people to wrap their heads around the code base before being able to figure things out. Volunteers are not likely to do this - you would have to pay a developer $100K USD per year for a year just to wrap their heads around the project because the code is so much of a mess, before you could even start making small incremental changes.

The most likely end result is that if it was open sourced, the code would sit there until the end of time with nobody doing anything with it anyway. Nobody could afford to pay salary for developers with no income source, and the Finale code is certainly a huge mess, dissuading any volunteers who might otherwise want to work on it for free.

There's tons of open source projects out there that sit there rudderless, abandoned. Open source isn't some magic thing. An open source Finale would most likely become just another such project - open but abandoned and with nothing happening. That scenario wouldn't help anybody.

My sense is that the reasons MakeMusic doesn't seem to be interested in open sourcing this aren't what you seem to think (profit, "making a cut"). Instead they know they would have to spend a lot of money to open source it because they have to vet the code since it would be illegal for them to release third party code and they would be sued, and even if they did all that and released the code, it's likely to become an open source project that just sits out there collecting dust with nobody really working on it.

one last sad attempt to milk the cash cow before killing it

Actually, I think Finale has done well by its customers in providing an upgrade path for the price of ... an upgrade. Finale was for all intents and purposes open source, as evidenced by all the plugins developed by Robert Patterson, Tobias Giesen, Jari Williamson, Nick Mazuk, Jan Angermüller, and others. But the longstanding issues that we have complained about for more than two decades are probably unfixable without 1) a ground up rewrite of Finale to 2) fix deficiencies Dorico has already fixed with the bonus of a 21st century UI. Other companies simply abandon their users, sometimes at a whim (hello, Apple?). MakeMusic, in my 2c opinion, has done extremely well to offer this path. Those most deeply injured by this are the veterinarians who have kept this cash cow alive all these years, the developers who have given Finale such extraordinary capability - and add Jason Loffredo to aforementioned.

Such an overhaul would require an import protocol similar to what we are already experiencing. 

A number of years ago, the source code was for sale to everyone in the notation app business. Everyone I knew at the time turned it down.

Open source takes money and organization to maintain. Muse Group has abandoned it in all but name as they work to make MuseScore 5 into something good enough to be the de facto notation app for Hal Leonard, the largest music publisher in the world—and Muse Group now owns them.