If you are doing the SPA thing correctly you must duplicate business logic (data validation). Also when your API just sends "dumb data" (JSON) to the client, you are forced to make changes to the backend and frontend in lockstep.
I've often seen this meme repeated in debates that js frameworks require you to duplicate logic between the server and client, but most of the logic isn't being duplicated between the client and server, it's being moved from the server to the client. It's relocation, not duplication. Instead of your rails/django controllers deciding what html to render, that decision happens on the client. In this model your server is mostly just an authorization layer, and an interface between the user and the database. Hasura, Firebase, Firestore, Mongodb Realm, and several other products have been successfully built around this premise. You might not like the thick-client thin-server model, which is completely fine, but it's a somewhat subjective preference. The only objective criteria you might use to decide which approach to use is performance.
Move to a JS backend and now every dev on the team can write client and server side code?
Switching to JS would require a full backend rewrite. The backend devs may not do well with the switch, so they may have let go two developers and needed to hire a new JS dev. The presenter may have been let go with that approach, clearly not ideal for the person driving the project.
You'd also need to address the perf issues. htmx clearly sped things up. Is the alternative JS SSR?
Since you are using hypermedia & HTML for your server interactions, you don't feel that pressure to adopt javascript on the backend that a huge javascript front end produces. You can still use javascript, of course, (perhaps in the form of alpine.js) but you use it in the manner it was originally intended: as a light, front end scripting language for enhancing your application. Or, if you are brave, perhaps you can try hyperscript for these needs.
This is a world we would prefer to live in: many programming language options, each with their own strengths, technical cultures and thriving communities, all able to participate in the web development world through the magic of more powerful hypermedia, rather than a monolith of SPAs-talking-to-Node-in-JSON.
Diversity, after all, is our strength."
Client side validation doesn't prevent a malicious user from sending invalid requests, but it can prevent legitimate users from sending invalid data to your server accidentally. In fact, if I see validation failures showing up in my server logs for something I know should have been filtered out via client side validation, I can mark that ip address as being potentially malicious and rate-limit their future requests.
And as a user I would rather find out about validation issues immediately instead of waiting for a network round trip to the server. If I'm typing in a password for example and it doesn't meet the website's length/complexity requirements, I'd rather know as I'm typing instead of waiting for an HTTP request to complete. That extra HTTP request is wasting the user's bandwidth and the server's resources.
SPAs were a workaround to slow CPU servers serving millions of requests in the mind-2000s. Client computers were faster, so it made sense to push UX logic there.
We've flipped things around. Servers are fast as hell for rendering HTMl. We can leverage that and re-focus the client on UI code that only it can do.
That and the panacea of being able to run same javascript on server and client.