Everybody has potential or they wouldn't be in the NFL. It's what you do with that potential that determines your stay, and/or whether you're just a JAG or go on to be pretty good. So far for Mills, he's not doing much with it and it looks like he's destined to be a career backup. Maybe still too early to make that call, maybe, but at some point you've got to quit making excuses and understand you've got to get better at the position.
He's had very little impact on anything as his 28.4 QBR would confirm. 28th worst BTW, and worse than he had last year, not by much (35.5) but worse nonetheless. Hell, Osweiler had a 49.3 as a Texan and that dude was terrible. Savage 41.7 in 2017, TJ Yates a 43.1 in 2011. Mills hasn't come close to any of that. Yet some argue so hard and make the excuses they do for a QB who hasn't shown any more than any of those guys have.