Even if most of the old users can adapt and follow the instructions to get the game working none of the new players most likely wont. If playing a new game requires messing with the advanced browser settings and/or which warns daily about the content being possibly a security risk they will just find another game. Player acquisition is very important in F2P games.
The downloadable version should however work beyond 2020 as it should be fully under BB control and works like any other regular desktop game.
I posted some thoughts in the test server version of this thread and BB_Alex commented:
"We do not have any plans to make a TSO 2 or port TSO to HTML5 or mobile. Both are not feasible.
But we will keep on developing the game."