Even if you ignore the history of companies like MS and Oracle what it comes down to is vendor lockin. If there's a chance a vendor will pop up in 3 years time with a huge bill, why take the risk when there are plenty of free alternatives.
MS recently made one tiny decision that was unfriendly to open source, it caused a huge outcry and a reversal within 48 hours on a weekend. I can't even imagine what the reaction would be in your scenario.
Besides, basically everything is open source now and you can't put that genie back into the bottle.
Microsoft's handling of .NET 's OSS community has been haphazard at best. Just a week or two prior to the 'dotnet watch' debacle, there were issues and concerns with the .NET Foundation that led to the Executive Director stepping down [0].
I bring this up, because in many cases the perception is that there is -still- lock in, just in a different fashion.
By that, I mean, if you Ask a typical .NET developer what they use, they'll probably say ASPNETCORE, EF Core, maybe you'll hear Hangfire, MediatR, RestSharp, or Dapper.
So, you've got a bunch of .NET devs that -only- know Microsoft technologies for the most part. Yeah there's some other stuff like MongoDb, Kafka, Redis, stuff like that, but It's not very frequent you hear about teams reaching out to other technologies.
It's very rare I hear people bring up Linq2Db, a beautiful* Micro-ORM that is best described as a type-safe, extensible SQL DSL. Or Websharper, a really-freaking-cool library that basically lets you transpile your C#/F# code into Javascript and/or Reactive HTML, complete with seamless server calls if you'd like.
You might run into some interesting things at different places. One shop I was at used MassTransit, which was kinda cool. I've wound up using Akka.NET a few times in the past, which has always been super fun.
The end result of this though, is the -perception- of what .NET Developers are like. And sometimes those perceptions are real. I remember the dev that felt Dapper was some sort of 'black magic' and would stick to writing DataReaders and or datatables by hand, and another that was so against the idea including Non-MS tech in a project that it wound up costing him his job; he insisted there was a way to get EF to do things in a performant way (answer: not sanely, and not easily the way the app was built on an arch level,) and refused to accept a PR that solved the problem with Dapper.
He wound up doing the thing I've seen a -lot- of .NET developers do; fight the Framework.
To be clear here, I'm not referring to the BCL. It's not always perfect (I'd love for an analogue to SSLEngine, please?), but it's -fine-. I'm referring to bits like ASPNETCORE, EFCore, SignalR, and Microsoft.Extensions.(DependencyInjection/Logging) where developers wind up getting in awkward tarpits around some weird edge case because of a business requirement or some other decision that, unfortunately, can't be undone.
Or are just plain 'well, that sounds sensible in theory' like "I would like to update N rows in an new status that are older than 1 month and set to overdue, and not have it be N update statements." Maybe EF does that now, but last I knew the answer was not really.
At my first 'Real' Dev job, we were a .NET shop, that often had to 'fight the framework' (it didn't help that we were on an Oracle Backend, which made -everything- more of a PITA before we discovered Dapper.) When the .NET guys hit one of these roadblocks, it would often take sprint after sprint of fighting to either have no solution, or have a solution that would render the app hard to maintain. The newer teams using Java? They didn't have those problems. We later heard they had 5 different ORM-ish libraries in use over there. At the time, a lot of the .NET devs kinda treated it as a sort of derision. 'hows somebody gonna understand it?'... But the Java teams delivered. It is also worth considering, maybe those were the best libraries to solve the problems that the app in question needed to deal with.
And that's kinda the 'mindset' that is a set of .NET developers that fit the stereotype; if it's not an app that fits their cookie-cutter world, they break down and can't understand it. In other words, they're afraid to step outside the box, which means they're less likely to think outside the box.
The typical 'litmus-test' of this type for me is a sliding scale based on their past/current experience with other languages and willingness to work with them.
* - I do some contribution work to Linq2Db, so my opinion may be a little biased.
[0] - https://github.com/dotnet-foundation/Home/discussions/39#
