I say “we” because I work in software which, past a basic level, involves endless arguing amongst colleagues about what the “best” language to use is for a particular project haha. Your question of why new languages is a really great one, with controversial answers haha.
What it boils down to is that every language is a set of opinions and compromises. Sometimes the assumptions that a language maker has just become less true over time. For example the language for making websites that was written in the 90s assumed that all you would need is a way to describe titles, paragraphs, columns, and some pictures. Websites do a lot more than that now, so people have come up with new languages for building websites.
A lot of it also comes down to people/politics. Since making and maintaining a language can be complex and expensive, it is frequently only taken on by large corporations. Those corporations are always doing their best to lock you into their ecosystem, and they use their language as a carrot. If you use our language and our services, everything will just work the way you want without hassle! By the way next year we are doubling the price of our services but whatcha gonna do about it, you already wrote a bunch of expensive code and hired a bunch of people that are only familiar with our language…pay up now!
Thats definitely one of the reasons Mozilla is developing rust, but not the only one. The goal of Rust is to make it harder to write insecure code. That requires more upfront work for the coder, and can be less efficient with computer resources, but provides more peace of mind that the finished product doesn’t have as many bugs. Thats a trade off Mozilla and Rust users think is worthwhile, but not everyone agrees with that.
In terms of vendor lock-in from language choice, it’s a big fat It Depends™ lol. Theres a ton of money in big tech, so there are a lot of clever people who work full time trying to lock you onto platforms without it looking to you like you’re falling into their trap, and they use a myriad of tools.
It is rare, but not unheard of for there to be license fees just for using a language like you said. (And sometimes there is even disagreement about who gets to charge licensing fees for a language: See the Java debacle with Google v. Oracle.) However typically the issue is more around being able to use the new features of a platform when a vendor introduces them. For example, Apple really wants you to use Swift to write iPhone apps. You can use alternatives, but then when Apple introduces the new iWiping feature that revolutionizes buttholes, you won’t be able to provide that feature to your users until your language of choice gets updated to support iWipes. That could take a year and means your app is behind your competitor, who just used Swift so Apple made sure they could support iWipes on the day the feature came out.
Another extremely common example is that entire businesses are run through complicated formulas in Excel spreadsheets. Sure, there are many other ways to run a business and keep track of the numbers, but most people start small and then the complexity of their spreadsheets just grows and grows and grows. Those people will be forever stuck paying for an excel subscription (that didn’t even used to be a subscription when the business started, but now it is for some reason!) because the complexity of replacing everything is scary.