Even if it was to be his magnum opus, it isn't like all of his ideas were completely disgarded (and lets be honest Nomura's not the type of guy who is afraid of changing things up himself). After all the team made statements on adhering towards Nomura's vision as much as they feasibly could, regardless of execution. Yoko Shimomura even acknowledged this when she commented on Versus XIII and FFXV's base story being very similar overall despite changes here and there.
In my perspective the situation really isn't as simple as Nomura being slowley yanked away for Tabata to seize control, and it has been highlighted before that Nomura puts a lot of trust into his co-directors while admitting to having flaws managing projects on his own. As for that VII Remake commentary, the timing is very ambigious as to when it happened, and we have to remember that 2013 was the year of S.E. facing company restructering among staff and such. Hence the Business Divisions being born.