So it's basically a quantity VS quality question.
I would say there are three points to take into account:
- Is the quality decreasing? Better not stop a franchise while its quality is improving. The unfinished franchises by Valve have their last opus as the best. These franchises feel like they were ended too soon.
- Are there replacements? I'm seeing God Of War and Dark Souls, which are my favourite franchises, Dark Souls is over and the soul of the first three God Of War is gone, and I miss both of these franchises, because they don't have replacements.
- Would being "the best of the franchise" make them excellent games? E.g. Pokémon games are pretty average, but they have Pokémons; the best of all Pokémon games may just be an above average game ^^"
I'm not sure how to weigh the three items above to make my decision ^^ I feel that the first item is the most important to me.