I'm assuming this account will be used for other things besides purely medical reimbursements.
Why would you assume that? No one has stated you should do that. Most have stated keeping your receipts so it counts as a medical withdrawal and you pay no taxes on it.
Because an investment of $6650 per year, returning an assumed 7%, is worth about $100K after 10 years. If you start maxing your HSA at age 25 and retire at 35, you're probably going to have a hard time finding $100K worth of medical expenses to get reimbursed for. If you retire later and let it compound more, it gets even "worse" (for some definition of "worse" where having too much money is somehow a problem...).
Not really, when you take into account that the "average" person is recommended to have $250,000 for medical expenses for retirement after 65. I fully expect to use my entire HSA for medical.
Okay, fine. Let's assume you'll spend $250,000 after age 65 on medical expenses. The only trouble is, that $100,000 that was in the account when you retired at age 35 has now grown to about $750,000 even though you haven't contributed any more to it. What are you going to do with the other half-million?
(Actually, I'm cheating a little on the math: I assumed the HSA contributions for a married couple, but the deductions for a single person. It would be better to assume you and your spouse would spend $250,000 each, leaving you with "only" a quarter-million excess.)
Well, a few options. First, assume that the loop hole for withdrawing medical expenses is not limited and I pull out "medical" expenses tax free as needed. Second, the $250,000 is for someone retiring now, not in 30 years. Assuming basic inflation I would assume a need of $500,000. And yes that is for one person so double that and you have 1 million, over the $750,000 you projected. And third, say you do have extra, is that a problem? It does not have RMD unlike a traditional IRA but otherwise it does operate as one. It is one more bucket of money.
So, assuming you are able to max out all your retirement vehicles, and have left over to put in a taxable account but only if you withdraw the HSA as you have medical expenses, I would recommend leaving it in the HSA to attend to your later medical issues tax free and a bit of fudge room with your "medical spending" money.
ETA: Also you are ignoring the need for medical expenses from 35-65 which can be a chunk of change.