The reason is simple. They want the recurring revenue of a subscription. The subscription will be cheaper than purchasing for most people, so it will be a no brainer (I think between $150 and $200/month ).
Elon will justify saying purchase is cheaper because the cars can go a million miles and last well past 10 years, which may be true, but for most new car buyers, they won't keep it nearly as long.
Once the subscription comes out, it's a no brainer for people who are waiting to switch because now they can pay the subscription. Did they waste $5k? Yes. Is there a better alternative currently out there? No.
For my math logic I'm assuming most new Tesla owners will keep the car between 4-6 years, and by the time we have a subscription, the price of FSD will go up to at least 15k.
I've read that the current take is between 20-30% have purchased FSD, and I would expect that to get to the 70-90% of purchases if you price it under $150/month.