First, these aren't kids these are adults who work full-time making millions in revenue for their de facto employers (soon to be de jure after another lawsuit or two).
Second, what coach really needs millions of dollars to pay the mortgage? As long as coaches are getting millions, there's no reason players shouldn't. In the pros a lot of players make more than the coaches. And yes, even in college baseball almost every SEC head coach is paid over $1 million/year.
Adults in the eyes of being 18, yes by law I suppose... At least you're admitting their sport is full time and school is a secondary concern and those high NLIs are not for tuition... So why not go directly to the "working world"? Because "most" pro leagues have certain requirements that stem from years of experience and a strong desire to keep their collegiate "farm system" viable. IOW, they're "adult enough" to realize they don't want to babysit some 18-19 year old (generalities here, there are exceptions) and they need the system in place even though the exception is hard to "pass up". The Lebron James' of the world are rarities and more often than not the gems wash out for one reason or another.
w.r.t. your second point - is there a coach in the SEC (or any major college program or professional team for that matter) that just graduated high school and gets that kind of coin without first proving themselves? Read a story recently about UNC Field Hockey which hired a just graduated player, but I think you can admit it's not the same.
I agree no coach "needs" millions of dollars to pay the mortgage. Are banks willing to underwrite a mortgage to someone that just graduated high school without some amount of work history and ability to pay over the 10, 15, 20, 30 years that the mortgage gets written? Apples and oranges type comparison. "Today" 1-mil may cover the cost of the mortgage, but banks have learned that doesn't mean anyone is going to pay their mortgage first before perhaps enjoying themselves instead leaving the bank (and others in this world quite frankly) on the hook. If the house is trashed because the younger adult didn't really value the property, then what?
If your argument is colleges shouldn't be "using" the athlete(s) to make bank, I have some agreement here - it doesn't equate to millions though. But let's face it, they put a lot of money into it, they do need some ROI. Why choose the SEC? For education or sport? They've built up their sport and brand thru years, it's not cheap. Stop thinking in terms of whats in it for me and I should get reimbursed for choosing to go to your school because my child is so awesome. If you've followed this forum long enough you'll understand the axiom of your child is the next greatest thing while they're being recruited. After that, the coach is looking for the next greatest thing and pointing out to your child who that next recruit is that will take his/her place. Just like what happened when your child was recruited. It's a self fulfilling prophecy / cycle.