I am a bit pessimistic about this … a Plasma chain can potentially have thousands and millions times more transactions per second than the Ethereum main chain. In addition to that, Plasma users may have much smaller average transaction sizes.
If I have $10 stored on a Plasma chain which I am using for micropayments, why should I care to validate Plasma transactions of unrelated users. Doesn’t this look like as a textbook example of “the tragedy of commons”. I will probably not care at all, since Plasma operator is a good guy and since I only have $10 to lose.
What about the spurious UTXO chain example that you brought up in the past?
Or about an example where Alice deposits $10, makes thousands of self-transfers over a year and then exits all of these UTXOs. It seems that in these examples one needs to keep long long history to catch the fraud.
If you say that essentially “someone will challenge but a typical user will not challenge”, wouldn’t you call the someone who challenges a de-factor validator. But if you gonna have de-factor validators anyway, and the security will depend on this small number of pro-active users. why would not you explicitly call them validators and economically incentivize them? ))
If a Plasma chain will be 1000 times faster than a normal chain, why should one validate this thing for free out of altruistic reasons, if even in the existing main chain there is a problem where the guys who validate things are not paid anything?
So it seems to me that a statement like “Plasma without validators is not Plasma” is a bit naive. Who cares after all, if introducing validators can make things more secure ?))