If I were you I'd probably just SD the card, only taking it out once every 3 months or so for a small swipe, then continuing to request CLIs every 6 months. Since it's an old line of credit I'd keep it open so that it continues (past 10 years) to positively impact your AAoA.
You could shoot an email to the Capital One CEO, which will prompt a call from the Executive Office usually within 2-3 business days, and sort of vent to them about your situation and history with CO. Since you've been a long time customer with flawless payment history, they'd at least be willing to hear you out and possibly something good could become of it. From what I understand CO doesn't do manual reviews any longer (although I'm in the middle of one right now as far as I know or was told anyway yesterday) so who knows what the result may be. I'd just be honest with them and let them know you'd like to continue the win-win relationship you've had for a decade+ at this point but without the ability to increase your CL you're not sure if that's a possibility and see what they say.