So far my modem has lasted 7 years (when we switched from dialup to high speed).
My modem is docsis 2.0 which continues to be solidly compatible. Comcast are slow to support docsis 3.0. I have looked at upgrading but we get very good speed without it.
Router: I have been forced to upgrade my router because my old routers died.
Wonder if you'll get the same performance out of your new Linksys.
Sure hope e4200v2 (N900) router lasts! Paid $200 for it! The reviews were mostly positive. Setup easy. Toss disk in and follow instructions.