>disruption in terms of retraining or having to constantly train new people
Ya, tell that to the short windows upgrade cycles, or you pay through the nose for LTS. A sensible open source approach lets you control the upgrade cycle, and support applications in the long term.
The real issue is that it takes a different approach. You need to hire high-skill programmers and managers to work for the state. And it's not just a take this off the shelf and use it approach. You really do need to examine the supply chain risks and work to support and expand critical projects.
As for java-script, use it on the internal pages and in-department web tools that you want. However if the site is providing public facing information or services, there can be huge accessibility issues raised by these client-side scripts. And even there it isn't banned, there just has to be an alternate way of accessing the same data.