Wouldn't you expect those defending the purchase to have believed he had the chance to be very good in the first place? Shouldn't you be asking who was saying his price was insane and he was never worth it are now saying how brilliant he is?
As for no doubting him, he is what he is. He's a 19 year old who is already a very capable striker. This isn't someone who signed and got a few lucky goals, this is someone who performed at a higher level than several very good players game after game, for the whole game, creating chances for himself, others, scoring, winning the ball, showing strength, turning players, shielding the ball, driving at goal, beating players.
Just about anyone can fluke a few goals in the first games then never get another goal again, but few play this damned well. This suggests this is simply his current level and there is no reason to believe he'll get worse as he plays more. How much better he gets is entirely up in the air, he may have peaked, he may eclipse Ronaldo... it's likely somewhere in the middle, this is effectively only his 3rd season and probably the first year he'll make 25+ league starts(barring injury), to be this good this early is insane. it would be pretty normal to progress and get better in the first few years of solid starts which he still has to come.