Of course they wouldn't, and they shouldn't. Releasing the source code doesn't absolve them of a responsibility to make sure the game is actually working when they end support. "We fucked over all our players, but here's the source code so someone else can fix it for nothing" would be a really shitty thing to do and they shouldn't avoid penalty for fucking over the majority of their players (and the unpaid people who will have to fix it for them).

On the other hand "we patched the game so it'll continue to work for everyone who bought it" benefits most players, and "we patched the game so it'll continue to work for everyone who bought it, AND here's the source code so others can expand/modify it if they feel so inclined" would satisfy everybody. It just shouldn't be a legal requirement.

Also keep in mind that in the UK system, if a petition reaches its 100,000 signature minimum in order to be considered for debate in parliament, that's only the beginning of the process. It doesn't just get put into law exactly as the petitioner words it. It goes through multiple debate stages, where the MPs consider all the options, and then the law gets written - and then it usually gets amended a few times. So I would expect that if this petition did lead to a change in the law, the resulting legislation would have considered multiple options for what "leaving the game in a working state" would look like. A surprisingly large amount of UK legislation on this kind of stuff sort of goes "this is what we want, but companies have freedom to choose how they will implement it".