I'm a regular Ryanair user and I doubt I'll be doing this for reasons that nobody has mentioned yet.

For example, I'm about to book a one-way flight from Brussels Charleroi to Poznan. The base price for the day I want to travel is 14.99 EUR.
For 28.50 EUR there is a package of extras - reserved seat, priority boarding, 10kg larger bag in the locker. I always take this package, it makes everything a nicer experience and removes the things that would cause me stress. (I know some people don't want to give Ryanair a penny/cent more than they have to, they're entitled to that point of view but I don't share it.)

If Ryanair Prime includes a free reserved seat (& that seat is a reasonable one) then that makes this package of extras less attractive.
If I would not take it then I'd still need to pay for priority boarding and the 10kg locker bag. This would either be separately (I don't know how much that is or if it's even possible) or still pay the 28.50EUR and effectively pay twice for a reserved seat.

If I still have to buy this package in order to get priority boarding and the larger bag then that effectively removes the reserved seat as one of benefits of Ryanair Prime.