First of all "his first season as starter" what does that even mean? What was he doing during his 2 seasons in Benfica and his first season in Frankfurt? You wrote this like it's a good a not bad thing that he didn't show anything special until this season.
Secondly you are describing this guy like he is a world wonder while in fact he is only one season wonder for now. If he performed like this for 2 or 3 seasons in a row than I would be more than excited if were in the race for him. IMHO 60 mil for ONE season is just to much. I sincerely doubt that European clubs will be all over him if that amount is correct.
Thirdly I agree that German league is better than Dutch but the level of German league has dropped significantly in the last couple of years. Paco scored 14 goals in less playing time and even Kramaric scored 13 goals in Bundesliga this season and I don't rate him at all.
He literally wasn't a full time starter and spent more time as a sub until this season. Why is it so bad that a player only broke out as a 21 year old? Literally none of Suarez/Lewa/Cavani/Salah/Diego Costa/David Villa/Falcao/Forlan/etc. were playing as well as Jovic is at his current age this season domestically, in Europa, and for the NT. And that is only a few, I could go on and on with names.
Would you be saying the same shtick of "it's a bad thing they didn't show anything until age 22-23+" about those guys?? Only very few strikers like Aguero/Rooney/Torres were rising stars at a very young age and ultimately turned into world class players, and 2 of them burned out to only have half a career at the top level. Only Mbappe is out there right now in a similar vein and we don't know what he's gonna be like down the line.
Your argument about Jovic not showing anything until now as if a player is a guaranteed bust for only breaking out as a 21 year old is just absurd. I don't know why you stick to it when it clearly doesn't make sense. Jovic's goal record wasn't even bad last season for the amount of playing time he got, scoring a goal every 120 minutes or so. If that's what you mean by utter mediocrity, I don't know what to tell you.
All I see is you being overly stubborn sticking to your guns about the idea that he's a Paco type player which was originated out of nothing but hot air and conjecture. I'm not saying he's literally the next Suarez, but he certainly is one of the best young strikers on the market right now and shows potential in a lot more areas than many of his peers. You seem to refuse to admit even that much.
I also see a BBZesque next level conservativism about finally accepting that a young talent has legitimate potential. With Arthur it was clear that the former was a great talent when he was leading his team to a Libertadores title as the best player in Brazil. With FDJ it was pretty clear early in the season even during CL qualifiers in the late summer that he was class as a midfielder and had the whole package in terms of skills. And yet BBZ was over here proudly beating his chest that he "accepted" their potential once both players were already dropping 9/10 performances against the likes of Tottenham/Real Madrid/France/Germany/Bayern/etc. Dude was 10 years late to the party and then tried to be sly and act like he was in the right about finally praising their legitimacy as prospects when they were already 80mill+ value players. It's just low level talent evaluation, and you shouldn't follow this suit.
That's exactly the type of shit that big clubs with bad scouting do and end up paying out the ass for talents when they've already emerged as stars. You guys just need to restructure your expectations for young players and reevaluate what you should be looking for on a production basis and skill basis.