I think there are a couple things going on here that can all be simultaneously true, but also has some shades of gray.
1. Is this the worst Purdue team in the last 40 years, I would objectively say no.
2. Coaching matters to a point, but players matter as well. Recruiting matters
3. You can build team depth in 3 years. You just can't miss on many recruits and have that depth.
Here is some context. Below is the 247 sports composite talent ranking for the years 2017-2019. The way to read this, it's taking the entire roster from recruiting and stack ranking the entire league:
2017
2018
2019
Purdue has more collective talent on their roster now than in Brohm's first two season, but it's still last in the Big Ten West. If you have talent issues at the wrong spots, it can really be magnified. You need strong QB play to win in Brohm's system, but also you have to block. Purdue should move up this list again next year based upon where they are in recruiting right now (4th according to 247). It's ok to expect more from a more talented roster.