The bigger concern about spending hundreds of dollars on new TiVo hardware and lifetime service today is NOT TiVo itself, but the ability and willingness of your cable company to support CableCards. With the FCC dropping the requirement that companies provide or support CableCards, they are no longer required to do so. Many people are finding that getting their provider to provide or activate new CableCards is much more difficult just a year after this change - what about 2 years from now? (That's why we left TiVo after 15 years, and you can read similar other long stories on this board.)
At some point, providers will set a hard end date on CableCard and refuse to pair them, and also moving channels to IP-only (hence not TiVo accessible) is already common on a number of cable systems. My suggestion is to keep using the existing TiVo hardware as long as you can, and expect to move on to something else next.