I've never thought about this. Sounds fair to have results dropped a percentage each week until falling apart for ever, but that would make the system extremely complex, and I don't think anyone would ever go for it. But it's an interesting idea which I like, nonetheless.
If you're interested, have a peak at the ranking system used in golf. They have a 13 week grace period where all points taken stay the same, then they drop just over 1% each week until exactly two years has past, and they drop opp entirely. Also they use average, instead of "xx best". That means it's always an advantage not having played much 19-24 months ago as those results will always be a drag no matter how good they were initially.
Golf also has less obsesion with the majors. A major win gives 100 points. A win in the biggest non-majors gives more than 70 (exact points depend upon quality of the field).
Do you think any of these features would improve the tennis ranking?