If Lowell had stayed with the series, I think it could have gone on a year or two longer than it did.

Possibly, but I think the actors were all ready to move on after so long though.

Oooh! Alternate timelines!

If Wings had continued with Thomas Haden Church after Season 5 or 6 or whatever, THC would have recieved a deal for a Lowell Mather spin-off similar to Joey and DiC would have pitched a deal with Paramount to create a Lowell Mather animated series to accompany the live action series. Billy West would have provided the voice of Lowell in the animated series.
The series Lowell would have run for five seasons and had established a cult fanbase simiar to that of Family Guy.
Thomas Haden Church would then go on to do mainstream movies.

That article about the last episode stated that Tim was ready to leave, and I'm betting that other actors were too. Seven-plus years is a long time to play the same character.

If Lowell had stayed, I wonder what his storyline would have been in the final episode. I guess he would have just stayed as the mechanic, since he stated in an earlier episode that he loved his job and Nantucket.