Some ramblings.I too feel that once a game has reached it's zenith, it is probably best to let it go...mostly. At least as a commercial product.
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So my suggestion would be along the lines of a readily moddable platform (XML makes this very feasable now) - and after the half life runs out - just do major bug fixes.
I absuletely agree on this. After a game is done, you shouldnt have to add mayor features to it anymore, and you can let it go. Especially if the last version of the game/expansions has an intrigate map-editor, and even better mod-editor.
However, I think a running and existing company, should never really relinquish patching of any kind. Simply to balance out certain factors, and better, remove certain bugs which always be lingering around. Oldstatesman pointed out perfectly how annoying bugs can be. Bugs seriously make you quit a game in rage.
In a company's view, while they won't see any immediate recompense in actual value of putting any effort into patching games, any company will be able to see this in the long run.
If there are consistent bug-fixes, till a certain point where the game is "bug-free", a game lifespan will increase dramatically, resulting in more sales of that product, and far better a very happy public. A company can make a very good name, and therefor will sell future enterprises more easily.
Alot of companies use the one-day fly approach. They release a game, patch it once or twice, and let it go. This all based on the assumption, the game will sell itself, and will live very happily ever-after in this world. WRONG!
A good example is E.A. They release a game, and let it go. Maybe throw in an expansion or 2, but never bother much about patches. One of the reasons, why I always hesitate in buying games made by E.A. While it gets harder and harder lately to dodge it.
Every company should take an example of Blizzard though. Blizzard hasnt got much titles in all the years it has been a company, but every single title has got so much attention, before and after the launch of the game, that they got a huge group of supporting fans. Again, you want this as a company. Because whenever you will release a title, it WILL sell out on the release day. And you barely have to run a selling-strategy, since your company's name and reputation is the selling-strategy.