Hmm, I wouldn't argue it is speculation at all - we can see from history that the professor's assumptions are deeply misguided. To explain:
Suppose you are a 51% actor in a PoS system and you wish to exploit your majority share. Of course, if you put up a big sign saying "I'M GOING TO ABUSE MY POWER" or do actions that have a similar effect, people would withdraw their money, stop using the system, and the value of your currency would plummet - meaning you wouldn't do that. And in every historical situation it is possible to find - people do abuse their majority share, even when there is a great deal to lose.
Example 1: business. Hostile takeovers in business often seem to come with huge risks in this way: you could take over a business, get a 51% share of its shareholding and then immediately make a huge price rise or begin trying to lower the cost of its products as fast as possible to make more profit. If you did that, customers would notice and would abandon the business - so businesses don't do that.
When any business gets a monopoly or performs a takeover, it keeps a low profile about any future plans for a while. Then it slowly, insidiously, begins to take advantage - by slowly raising prices or abusing its power, until by the time people are largely aware of it, they are unable to take any action. Apple has done this with its iOS ecosystem, Amazon is doing it right now, and many other companies have done it in the past.
Example 2: politics. There is a lot to lose if you try to take over government and abuse power, but there's also a massive reward - and it's surprisingly easy to succeed really. 1930s Germany and 2015-present USA are good examples of how you can perform a takeover in front of everybody and succeed; you can only "write out the bad actors" if they have no influence and there is consensus to write them out, and any competent bad actor will wait until they have sufficient influence before making their move.
I wrote more about this (and a similar section to the stuff I wrote above) in here. https://antsstyle.medium.com/why-nfts-are-bad-the-long-version-2c16dae145e2
Specifically, the "False sense of security in PoS and DPoS" section, but also in some other ones.
The main issue here is not that these ideas don't present an improvement over centralised systems, but that they in fact are easier to subvert - partially because of higher motive, and partially because should you succeed in subverting them, there is no recourse and the system is broken forever.
