What people don't realize with GTLD's is that this is not a new thing we have the data from .travel .biz .info .mobi if launched in 2015 all of these above extensions would have had incredible buying hype, and would be considered top tier with the obvious premiums. The only thing is I can buy a one word .biz for $1xx, same with .info, .mobi, etc...
This does not mean GTLD's will not have that random top tier sale we tend to see from time to time, people do think outside the box, but to carry a diverse portfolio to find this out at this point when the extensions are not spending marketing dollars educationg consumers, nor is there any media buzz anymore as their once was when they first launched. Not to account that when these premiums expire the registries are simply reserving them, and putting a premium price on them essentially becoming the domainer. So really what can you do other than to try to hit hot trending names, and topics, or sell off your good inventory, being careful not to spend it carrying dead inventory.
Any invested gtld owner if they are honest will tell you gtld inquires are down from 2014 2015 2016.... with the last batch of releases if you want to own solid terms you had to pay high EAP, or you have to continue to pay high premiums. Donuts was the most fair operator, and now under previous ICANN management they are raising prices, and really sticking it to domain holders who own larger portfolios.
There will be people who continue to hit these one off sales, and registries will still report those big sales from their platinum portfolio, but you have to be realistic are those odds in your favor, or against you?