That's a good estimate as far as full 1st party support, but "weak support" with occasional 3rd party games and firm updates, will probably go on for 2-3 more years. The idea with "EoL" is to release the exploit when Nintendo stops updating the firmware. That's a tricky thing to estimate as they never announce when they're ending support for firm updates. What I would do (if I believed in EoL releases) is to wait until the 3ds has stopped production. That way, anything you buy in a store or online should be on exploitable firmware.