This is a first for WoW, seasonal classic. Nobody is saying there wont be a TBC SoM, or that there will be. Being this is a seasonal “Game Mode” you can call it i guess, it makes sense that things are reset seasonally . At least they are rolling the characters into Classic Era servers after SoM is done, giving you the ability to continually play your “Seasonal” character, even if its not in the SoM mode.
It would be hella redundant to drop a seasonal mode, then drop the expansion for it and extend the season. They are basically… AHEM… LITERALLY recreating Diablo 3 Seasons for Classic WoW. Nobody should be the least bit surprised they wont extend a season or give the option of TBC SoM.
What they’ll probably end up doing is dropping a TBC SoM after the first season to run simultaneously with Classic SoM (if either are even viable.) and having people once again, start fresh in the TBC SoM or Classic SoM.
The idea of Seasons is for a new, fresh experience every season with some changes & a theme (i.e. Diablo & Path of Exile.). Its not to do what they already did with Classic WoW by moving on to TBC.
If you want to keep your character, just x-fer to Classic Era or TBC if they allow it. Other than that, just create a new character every season and enjoy the new, fresh experience they are trying to provide. This first season isn’t much (EXP increase, Raid Changes), but in future seasons we could see some really cool stuff, for example, class changes from spells, all the way down to talen trees, or even NEW dungeons & raids that fit the feel of classic, which all would be better than moving onto TBC classic from a dope season.