While .mobi domains still have traffic, I see no potential in the extension as a domain investment.
The TLD was a situation not exactly similar to anything else before or after. A consortium of tech companies, in the lead up to smart phones created the widespread expectation that each site would need its regular domain for computer users and a .mobi domain for mobile users. As such, and with projections for big numbers of mobile users, the .mobi would have guaranteed use and high value.
When Apple, and I believe one other, walked away and instead urged responsive web design, the need for .mobi almost immediately evaporated, and domains that had sold for big amounts in the auction mainly resold at much lower prices, or never.
While I understand those who feel it is relevant to new gTLDs (it of course was never part of the new gTLD program) I don't see it that way. No new gTLD can make the claim that you NEED this exact dimension (possibly might argue .app comes close), but rather are only valuable in terms of how the elegance of the name, the value of the keyword and match, and strength of the registry compete with other alternatives.
I suspect that the new gTLD introduction would have been much more successful had .mobi not come along just prior and so many domain investors lost money on .mobi, but really it is different.
Anyway, in short I would never consider a .mobi investment except possibly in the case of a hand-reg fee for a word where .mobi is a logical across the dot combination. Ignore all high priced past sales of .mobi - they were from an era that never came to pass.
Bob