Well (as a person who is heavily invested in new gTLDs) I can tell you following:
As for 2018, imho:
a) New gTLDs are not yet mainstream, but are heading to this direction - you can like it, or not.
b) Number of offers/sales are getting larger year by year - I can feel it myself in my portfolio. End users like unique and cool names.
c) We see some record sales of new gTLDs publicly announced, and this is just a beginnig - I would not be surprised to see some new gTLD sales larger then 1 mil until end of 2018.
d) Registrars, registries, ICANN, new gTLD investors
AND END USERS - they all find gTLDs beneficial. No end user will ever complain that there are more options to name their business. If they do not like those options, they can ignore them, no problem.
e) If you builded large .com or ccTLD portfolio over the years you can naturally have some issues with new gTLDs - it is not fun for them to see part of the money redirected to new gTLDs. So this is the only group of people who can complain, which is fully understandable.
f) New gTLDs are just another investment instrument in domain space - as for private investors, most profit will be done, imo, by pre-mainstream investors. Logically, no one can seriously expect to get great name for $1 AFTER they becomes fully mainstream. In the process of investing there will be (are) some losses too, as everything is still in process and we need to learn everyday.
g) It is not true that all good names are hold by registries as stated by one of OPs above - you can see TONS of amazing names in private hands, with regular renewals or very reasonable smaller premium renewals attached to them - people who do their analysis and work are doing well. When you study in WHOIS who owns great gTLDs combos, you will find people from all around the planet, PLUS some clever and rich .com investors as well, who have diversified their domain portfolios.
Just IMO
