There is one huge difference with the renewals of .com domains compared to the renewals of the new GTLD's. Which is the new GTLD's are much more expensive to renew. This makes the holding of marginal domains unlikely with the new GTLD's. Couple that with the withholding of domains and premium prices, and the best way to play this market is as a Registry. Which in itself explains why many of the old domainers now own Registries/Registrars. Also with the new GTLD exposure still being very limited, compared to old TLD's, it isn't surprising you have taken this position. It's my position also. Although I have a dozen or so new GTLD domains. As a Domainer most these new GTLD's are not so attractive, because if you cannot find your domain in one GTLD, you might find it in another. I personally don't find much to my liking with this entire scenario. But I have dabbled in a few.
You make some very good points. However, the registries have to manage thousands and thousands of names. And they're only human, which means mistakes will and have happened. Grave mistakes. A lot of premium names have been given regular prices or very low premium prices.
If you listen to one of the recent talks on DomainSherpa about gTLD (DnSeattle event etc) you will hear one of the registrars confirm that some businesses are willing to pay 5,000 per year for certain premium names. Just let that sink in for a minute. And apparently that is not rare, it's a normal occurrence and sales are often in the mid 5-figure range. And as a business owner, I would do the same if I were in a business with a high profit margin like insurance or real estate.
So, if you find names that have a regular renewal fee but are in fact a premium names, you got a winner.
Find the low hanging fruit. Just dont get too much into it. Its easy to waste money on names that will never be worth anything because of excitement (happened to me a few months ago, wasted like 500 on a bad extension).
You need to keep a cool head and put in a lot of research before hitting that buy button. I believe if you follow that concept, there's A LOT of money to be made, not just by registrars, but individuals who do their homework.
Also dont invest into extensions run by random registrars. Donuts Co is the most established gTLD player. Stay away from those consumer extensions like fail, wtf, xyz, .. complete waste of money driven by marketing.. which will run out eventually.