He took over his first club. Was sacked for Sarri. After Sarri improved little, he was reinstated almost pulled off a great escape to keep them in Serie B.
He then won promotion to Serie A with Bari before going down with them.
After the Atalanta woes he promoted Sienna to the Serie A. After promoting Sienna he became Juve coach.
How does that make Conte good enough for Juventus, though? What makes Guardiola, Zidane, and Conte more acceptable appointments in this case? They won their second divisions, but they never proved that they could manage in the top tier. Those appointments were no less a gamble than Potter would be.
Would you have found it acceptable for Manchester United to approach these managers just because they got promotion from/or won their second-tier league? I highly doubt it. Hindsight is clouding your view if you say that they had shown more than Potter up to that point. Their first clubs were all at a low tier.
In fact, Potter had his first team playing at a higher level than any of them. When you look at the clubs of these managers, and the competitions they played in, only Potter was involved in a European competition with his first club. Ostersund were in the Europa League, having navigated the qualifiers. They knocked out Galatasaray, got through the group stages whilst managing a draw with Athletic Bilbao, and they managed to beat Arsenal in one leg of the knockout stages.
But there is more to Potter's management of Ostersund (2011-2018). When Potter took over management of Ostersund, they were a club stuck in the fourth tier of the Swedish leagues. Potter managed to get them promoted up to the top division (3 promotions in 5 seasons), and won their only trophy in history (Swedish equivalent of the FA Cup). All this with a club from a town that has a population of 49,000. That is an incredible rise. Is that not showing that he was exceptional at that level?
At Brighton, he has them playing excellent football. Now, people will look at position last season but this mostly comes down to their poor finishers. The game against Manchester United at the start of last season was a prime example of what they were like. They completely battered us but their forwards couldn't score enough goals. Literally, the main difference between this season and last season is that their players are actually scoring from the chances they create.
Brighton are punching above their weight under Potter. Just look at the net expenditure below.
Net spend of clubs around them in the table since Potter has been manager
West Ham - £130.82m
Manchester United - £295.04m
Arsenal - £277.46m
Wolves - £83.7m
Brighton - £59.89m
Tottenham - £193.86m
Everton - £80.68m
And this doesn't include the massive investment these clubs, surrounding them, had before Potter was manager of Brighton. That difference is a lot bigger.
So, in the end, he is massively overachieving with Brighton and he turned Ostersund into a top club (in the Swedish League) in the time he was there. So, how has he not shown he is great at the level he is managing at?