Coaches jump around much less often than fans think they will.
Brohm is unlikely to jump to Louisville in a lateral move.
I could buy your point of view on that except for these tidbit facts:
1) Jeff Brohm played at Louisville, was born in Louisville, got his first coaching start in Louisville. His family is there.
https://en.wikipedia.org/wiki/Jeff_Brohm
2) Louisville is the 29th largest metro in the USA. Just under 800.000 people live in the metro area by its last census. West Layfette, IN around 42,000.
3) Louisville has a much better quality 4 hour recruiting base of Ohio, Indiana, Tennessee ( To Cincinnati: Less than 2 hours; To Indianapolis: 2 Hours, To St. Louis 4 hours; To Nashville: about 2.5 hours; To Columbus; 3 Hours) And Atlanta metro in about 6 hours away. Purdue's last two recruiting class has as many or more Ohio, Georgia and Kentucky kids
each year as Indiana (home state) kids. Rondale Moore is from.... Louisville, KY.
4) Purdue is always going to be a basketball school. Football is a cycle at Purdue. Spread offenses are more B1G East and require more WRs with seperation speed. Pro-style offenses are more dominate in the B1G West division. Purdue is more competitive than Hazell time there but Purdue will not be top dog with Iowa and Wisconsin around unless Brohm become a superior recruiter. He is good but not that good.
5) Louisville is now in the ACC and being a part of a P5 conference meaning larger TV audience, bigger contracts, better facilities, fatter paychecks. Louisville is spending money on its football program in upgrade. Serious coinage. Louisville can be an upper tier annual football program in the ACC. Moving to Louisville from Purdue is still a step up the ladder if you want to have a legitimate shot at better bowl games and maybe a CFP spot. I don't see the move as lateral if it get you closer to your goal.