My understanding of the idea is this:
1) Using distributed code, the operation of which is assured by code on a blockchain based system like Ethereum, lending and investment can happen without the intermediation of banks and capital markets gate keepers. The savings from cutting these layers out can be shared between the supply and demand side of capital.
2) On a temporary basis only, some element of the rewards of contributing external capital, computational power, or other resources to a particular pool of capital are "extra" on top of the core capital allocation function of the pool. These can be structured in various ways but essentially the idea is to bootstrap money into the pools through a reward system that declines over time.
3) If the core proposition, that there is gain to be had from the disintermediation, is true, then at some point pools reach a sort of "ignition" point where the pool exists for that purpose only and the temporary bonuses are no longer required. This would mean that despite structural similarities, these are not Ponzi schemes since there is an eventual state reached were a real function is being performed. Some people will stop contributing to a pool as the rewards taper, but that won't matter because most of the money is now in there to be lent out for profit.
4) If that core proposition is not true, then they are Ponzi schemes because all the growth is coming from the rewards.
My problem with (1) is that these are already relatively low margin activities so how can there be enough disintermediation to go at once you account for the default and scam risk on the side of the borrower? It isn't the case that capital is expensive right now, tech investors are giving it away like its going out of fashion, headline borrowing rates are incredibly low, PE is going crazy buying everything. It's also striking to me that the promoters of these DeFi schemes spend so much time on bringing in new lenders but very little seems to be done on the borrower side. Surely if you're building a sustainable capital allocation business, you need a pool of borrowers? Ideally one in a business where they can provide substantial collateral for their loan, support high returns, but somehow can't access other forms of finance (but not because their business is illegal). That would seem to be a rare commodity so I'm surprised not to see pools fighting over access to these borrowers.