To emphasize, I agree with everything you've said, but I don't really think "code sharing" is the right metric. The important boost from my perspective is that

To emphasize, I agree with everything you've said, but I don't really think "code sharing" is the right metric. The important boost from my perspective is that it makes it much easier for front end devs to unblock themselves for relatively small issues, and it also makes it much easier for any dev to debug when they can easily follow the code through the whole stack. E.g. in the past I've seen it take weeks (that is, one or two sprints' worth) to add a teeny bit of data to the front end. Reason being the request had to go to the back end team, then it had to get prioritized and added to the sprint, yada yada everyone's been there. When everything is in the same language I've seen front end devs be much less reticent to just make the change themselves.