I wouldn't call him a lock just yet, for all we know in the future, a franchise like For the Frog the Bell Tolls or Mach Rider could get a revival by Nintendo and become a huge success over Murasame Castle.
I never said he's a lock. I said that he's all but assured
if he is revived in a new game. That in itself is a pretty huge
if.
Sakurai's rejected Takamaru because western fans aren't too familiar with him and he hasn't done anything significant in years, right? If he comes back in a new game, there we go: Takamaru's now a big boy.
Even if another old-school series was revived between now and the next Smash, why would that make a difference to Takamaru?
Punch-Out!! and
Kid Icarus both experienced successful revivals between
Brawl and
Smash 4. Hey presto! Little Mac and Palutena (on account of Pit already being an established veteran since
Brawl). Takamaru and Mach Rider / Prince of Sablé / all three? Why the hell not?!