You guys are getting a generous amount of time if you can go 15 years before a Poly Boot / Auto-Caulk / 3-in-1 / "Oatey" fails due to heat cracking.
(I list all these names because that's what they go by here; unsure about up North... plumbers typically call 'em Oatey's because one variety is made by the same co. that does PVC glue).
In my area of Texas, these pieces of junk last about 9 to 10 years tops before dry rot sets in. Prior to the complete cracking to failure, the collar starts to contract & a white band shows around the pipe to boot joint. Plus, the paint never really holds & you start to get an ugly stack showing on the roof.
Another problem (down here, anyhow) is that one of these with the plastic foot will usually suffer heat & the base tends to flex up on the corners, thereby pulling up your nails (this is especially problematic if the nails used are too short).
I've never had any problems with squirrels gnawing on mine, but then that may just be good luck up 'til now.
To me, one sign of a good roofer is someone who specs lead Jacks on all their projects; 3 in 1's are for production builders & cheapskates.