I agree that maintenance shouldn’t be feared but you do need to be cognizant of the risks of being shoehorned. If you’re “the cold fusion expert” this will probably not be rewarded by chances to work with new things (or often even the replacement for that CF system) because they probably have a long backlog of things you can do faster than anyone else. If it pays well, that has upsides but only as long as that system is around: you shouldn’t assume loyalty when they replace it and it’s definitely cause for concern if nobody else in the area is hiring for those skills since you’re one corporate restructure, merger, bankruptcy, etc. away from having to retrain in a hurry.

I think this is also a good area to contrast the two things listed: tons of places have jQuery, it’s still actively developed, and it’s relatively straightforward to replace incrementally whereas CF is proprietary orphan-ware prudent shops have been migrating away from since the late 90s, when it became apparent that they were only indifferently supporting it, so you’re one unpatched bug or Adobe management whim away from a messy forced migration.