Alchemist is still up there to this day. Madlib too. I don’t think it’s a magical well of talent and ability that suddenly dries up because an arbitrary time frame has elapsed.
I think for some producers they get money and a lot of other distractions which impact their craft…
Partying, drugs, alcohol, women, having kids, other business interests, lawsuits, stalkers, a myriad of other pressures of fame. Straight up mental health issues, and also general health / disability problems as they get older (you just wouldn’t necessarily know it as most are invisible).
For Dre it could have been a number of the above reasons.