Speaking personally, I have enough to buy P4 at full price right now.
I started in May as a trickle of posts, and June+ I posted more / got more reactions. And here I am. And yet, I still haven't bought P1-4. Why?
Future proofing.
It'll take me 2-3 more months (ish, not counting posts/reactions) to get P1 using the interest only , and P4 via interest only in 7 months.
Again, if they actually add P5, or move up P4 to P5 to protect the site (which was the point to begin with), I'm not starting back at Square 1.
I almost spent it all when I was in range of P4 with the biggest sale, but I knew I'd get buyer's remorse.
I got Mem+. It's all I need for now. But that's just me. Everyone has to take their own path.