It can be tempting to stand on an ivory tower and proclaim theory, but what is the real world cost/benefit? Are you building a NASA Shuttle Crawler-transporter to get groceries?
That's an important question. The real world costs of fulling understanding the behavior of a system like Redis, amortized over all the users of that system is likely to be very low. That's a huge benefit of using well-tested pieces (whether they are Redis, Cassandra or Oracle) as the foundation of systems.
The important question to ask is what you are losing. Losing availability in these complex cases is acceptable for the vast majority of applications. You can save a lot of complexity and cost there. Losing safety is much more of a problem, because the effects can be very long lived. Once you have put inconsistent data into your database, you are in a world of hurt for potentially a very long time.
I think that it's actually cheaper in the long run, even for small systems with small goals, to build on top of safe components. The costs of writing code to deal with the shifting reality of inconsistent data is higher, as are the costs of not writing that code.
I don't think that proving the safety of protocols, testing those safety properties once implemented, and understanding failure modes is "ivory tower" at all. It's just good engineering practice.