30 years?! That's amazing. I have read that the life expectancy for people with ET who are on Hydroxyurea is about 86. But, you're in your 60s, so...
I'm 74 this October and have had ET, treated with Hydroxyurea since I was 62. I found a old medical record of mine that clearly shows my platelet count was over 400,000 in 2004, when I was 52. It had been circled by a doctor or a lab tech, but no one had ever said anything to me about it. So, 10 years later, after being hit by a bicyclist, my bloodwork showed my platelet count was in the millions that day!