Now that we have Stubbs out of the way, let's take a closer look at this.
Of course, at the moment we can't know for sure about anything. No one can say Fonseca will be a better player than any of the guys you list. But there are reasons to think that he might be. One could easily argue that right now, at 18, he is certainly a comparable player to those guys at 18. Here is each of their rankings going into the Australian Open at 18.
Ruud 217
Tsitsipas 206
Rublev 185
Krygios 183
Mensik 142 (just throwing him in)
Fonseca 112 (now up to 99)
Rune 99
Zverev 82
So he is clearly in the mix. He also lacks a lot of that group's weaknesses: Ruud (backhand), Tsitsipas (backhand), Rublev (variety). Krygios (head). To me, his most apt comparisons are Rune and Zverev. Zverev looked great at 18, except he didn't have a volley and he could be frustratingly passive when he should press the issue. Neither are problems for Fonseca. Also, Zverev is probably the second biggest head case on that list. Not a problem for Fonseca, either who possesses the elite skills but not the self-limiting flaws.
I think Rune is the best comparison at the moment. Holger definitely flirted with first tier status before his extended slump. His ongoing coaching situation may be his Achilles heel. Fonseca possesses many of the same skills, but is more intelligently aggressive than Rune, and doesn't have a dysfunctional relationship with his parents. Plus, he has potentially a more killer backhand than any of those guys, save Krygios. I don't think Fonseca is a sure thing to enter tier one, but I do think he is the best bet since Carlos.
As for his loss to Sonego, I wouldn't read too much into that. Let's see how far Fonseca moves up in the rankings by the end of the year. I predicted he will break into the top fifty this year which would be immense for a guy playing his first full year on the ATP tour. If he has a flaw, it's probably endurance (he looked like he was getting winded by the second set with Sonego before coming back to somehow take the fourth set). Exact same problem with Mensik last night against ADF. They are young and they will get fitter and stronger.