I'm giving a new answer because I suspect my previous answer was based on a misunderstanding of your question, but I'm not sure (so I'm not deleting the previous question right away).
I think you meant to ask
What are the limits to increasing the subcarrier count in an OFDM system? It seems like as-long-as-possible OFDM symbols are advantageous.
And the answer to that is time-variability: If your channel impulse response changes during the length of a symbol, your OFDM system loses the "O" aspect, and even if it didn't, the subcarrier symbols would experience a loss of SNR due to changing channel.
So, the length of the DFT is inherently limited by your channel model not being stationary. It's very rare that a multipath channel is constant - if there are many things to reflect from, chances are high something changes. For example, You can use very normal 2.4 GHz WLAN OFDM signal reflections to sense the movement of a human chest while breathing; generally, OFDM radar very much is a thing, exactly because OFDM systems are exquisitely sensitive to changes in environment.
So, your OFDM symbol duration must be shorter than what you consider the coherence time of your channel.