Yes, absolutely correct.
But what you don't seem to follow is the cost in administration of sending out many small payments.
This means that google have to set a limit (they've chosen $100) to make it worthwhile, not only in terms of having the benefit of cashflow (as you quite rightly point out), but also to keep the profit margin high enough that they can offer subscribers a reasonable PPC.
The cost of that administration, if they sent out $15 cheques every month would cost you, the webmaster subscriber, more than $18 over an 8 month period I would bet (you don't actually think that google would pay for the cost of sending out large quantity small value cheques?).
So you are actually better off forgoing the interest to google, and getting a better rate of PPC.
Who cares what google makes - what is more important is what the webmaster makes from the system.
We will only know for sure google's costing structure when they say what they will be paying PPC - or subscribers comment on that.
I would add that if you expect to only get $100 every 8 month period, its surely not worth you bothering to subscribe to Adsense anyway?