> I just happen to work in a field almost nobody else knows anything about, so it's hard to find candidates to fill positions
That's a strategy that doesn't come without its downsides, but in the short and medium term it can work very well - and it appears to have for you. If you believe those skills will be needed for the remaining duration of your career and you also believe it will remain a field that "almost nobody else knows anything about", then you don't have much incentive to change your strategy and approach, do you?
I say life is short and it appears you've found your golden ticket, so go enjoy life and don't worry about this nonsense of maintaining your "coding for interviews" skills.
That's a strategy that doesn't come without its downsides, but in the short and medium term it can work very well - and it appears to have for you. If you believe those skills will be needed for the remaining duration of your career and you also believe it will remain a field that "almost nobody else knows anything about", then you don't have much incentive to change your strategy and approach, do you?
I say life is short and it appears you've found your golden ticket, so go enjoy life and don't worry about this nonsense of maintaining your "coding for interviews" skills.