The exact opposite of this is true.
In the near future, you may be right, as more money going to milking the current game means less money going to the next iteration, but, eventually, he's right. The more profit that they make on the current game, the bigger budget that they'll allot for the next iteration. That's the case with movies. If a movie has a $50M budget and grosses $200M, the sequel might get a $100M budget, and if it grosses $500M, the next sequel might get a $200M budget with the hope that that extra expenditure might help it become a $1B film.
The flip side of the coin, however, and which is probably not the case with movies, is that it may persuade them to go more slowly. Because the budget will be so big and expectations will be so high, Bethesda will want to take their time to make it as strong and perfect of a game as they can. There's a great deal of risk in that endeavor, while there's low risk in milking the current game. So, the success and milking of the current game may mean that we'll be waiting a long time for a sequel, but, when it comes, it'll be very ambitious.
Elder Scrolls 6 will take a very long time. The engine they use to make those games is just way too old. It was good in its day but we're talking about something that from Morrowind to Fallout 4 changed very little outside of visuals and slow mo. And even in that case, the visuals in fallout 4 looked absolutely awful compared to other games released at the same time and performed badly relative to those visuals. Don't get me started on the load times.
If its going to be done right, I'd expect it to be at least 2-3 years away.
That's what I think that they're doing: creating a new engine. They like to debut new engines with Elder Scrolls games. Morrowind was the debut of the Gamebryo engine, and Oblivion and Fallout 3 merely used an upgraded version of that, while Skyrim debuted the Creation engine, and Fallout 4 merely used an upgraded version of that. It stands to reason that they're working on a new engine for ES6, since that's their pattern and they need one, especially for next generation consoles. They might be able to plausibly deny working on ES6 at the moment, but they're likely deep into crafting the engine that will power it, so they're surely working on it, in a manner of speaking.