There is a difference between "a few more years" and "forever."
I have a client that is still using SM v11 (40 users) and it runs very well indeed*; and I know of a couple of others on v11/v12 that have very few complaints. Another client is running a several-years-old install of hMailserver +Squirrelmail (12 users) under Windows Server 2012, which needs close to zero maintenance.
Both of these clients use Symantec Email Security.cloud for incoming/outgoing virus/spam/malware filtering, so no detected bad incoming emails touch their mail server. This keeps traffic off of the WAN circuit, and lowers the maintenance overhead for the entire email system. In SM, it removes all that anti-spam etc. system and maintenance overhead (the cloud service required maintenance is astonishingly low, and actually getting better still).
SmarterMail v15 incorporates all the accumulated knowledge and patches to date, with that familiar UI. There's no reason that it shouldn't/couldn't be kept going for some time now.
As I and others have stated, forcing us off of that UI will quite possibly cause us to leave SM. That said, "the cloud" continues to beckon, and most of us will end up there sooner or later, for better or worse.
*Are they using all of SM's features? Like most users, NO. They use email, and some use the calendar. Administratively the archive feature is very powerful and good to have. That's about it.