So this is not really an altcoin but just an alternative client for mining exactly the same, with the only difference a different base-system for numbering the amount of coins... The only thing different is the display in the client then, right?
It really is an altcoin, though your understanding of the nature of it is correct.
One does not need to do things the wrong way (making a separate blockchain for no reason) to make an altcoin.
So why would miners adopt tbc ...
They don't have to. TBC uses the same blockchain as BTC, so both benefit from the same mining work.
That is, TBC miners benefit BTC, and BTC miners benefit TBC.
...if it really has no benefit or innovation over btc?
It does, as explained in the OP.
From my understanding, there is no benefit aside from saying a = b so mine a or b and its the same thing. So again, why mine tbc if it's the same as btc? If I'm mining btc already, what tangible benefit is there to switching to tbc or vice versa?