By commercially viable investment presumably you mean one that will pay for itself through additional revenue over a twelve to sixteen year period? If so it is probably true that building a new stadium at present doesn't meet this criteria, but that is looking at it in isolation from its function and doesn't take into account its capital value.
It seems to me that Max has not put a foot wrong so far with his development plan for the Club, and the sequence of investments up till now appear to have been in the right order to create a well established Premier League team. The emphasis on building a young team capable of flourishing in the League for a five year period, which hopefully is what is being put together at the moment, will allow Max to invest in the capital infrastructure that should go with it.
Building a new and bigger stadium before the team is ready is the wrong way round for us. If the team can be consolidated and successfully renewed over an extended period of years, then the requirement to make the new stadium financially viable in the way that normal commercial developments are measured does not apply because a well established Premier League team with a new fit-for-purpose stadium is a going concern that can be sold to new owners for an amount sufficient to repay all the investment in it made by the current owner, rather than just relying on revenue to do so.
Looking on from outside, that seems to be the thinking behind the current development policy.