The thing about arguing whether the likes of Bottas are any good is that you're actually looking at maybe a single percent or less. Bottas is quick, no doubt - he's outqualified Hamilton on occasion and shown bursts of true pace in races. In that regard, he's like Rosberg before him. Rosberg could be lightening fast at times and took poles off Hamilton on many occasions, ran away in races too, at times.
The difference is that the "last percent" guys - the Vettels, Hamiltons, Alonsos, Schumachers (well, Michael) - is that they seem able to find that pace more consistently and can run it for longer than the likes of Bottas, Rosberg et al. Look at Rosberg: he admitted that he basically had to give everything he had, to the exclusion of everything else in his life that season, to beat Hamilton to the title. And then he retired because he knew he couldn't do it again. Heck, even that level of intensity needed a bit of luck in Hamilton's engine failing in Malaysia. That's the reality of taking on the very top tier drivers in F1 - you have to be absolutely on it every time you drive because any gaps, any dropped chances, will be seized upon and taken advantage of without mercy.
Hamilton, Vettel and the like just seem to have that extra "spark" (for want of a better word) that drives them harder for longer. It's not skill - all of the guys are skilful in the extreme - it's just something in their make up. It comes across in many cases as an arrogance, a "I deserve to win everything" attitude that many find distasteful or difficult to comprehend. But it's what sets them apart from the oh-so-nearly guys.
Max and Danny have the raw speed to compete with Vettel and Hamilton over a lap or even a race. Do either have that "spark" that allows them to do it for a full year? That's the interesting thing for me.