This is always tough, for Trello this was trello.com/templates, it's something we had thought about doing for a long time but we didn't want to be overly prescriptive about how people should use our product. That being said, we got a lot of inquiries about templates - and a LOT of other tools provided templates - so it became something that felt like we should consider adding. Like I've said in a few other answers, I think the best way to convince internal stakeholders that something should be a priority is with data.
Again, this can be a combo of showing that a feature is table-stakes amongst the area your product plays in, but something even more impactful is collecting feedback from support tickets, data you've gleaned from analyzing user behavior, tests you've run by sharing certain types of content our blog or through your email program that have led to higher engagement. Anything you have that can be an indicator of why a feature you may be advocating for could benefit your users and the company.
The other piece of this is figuring out how your product can do it differently. For Trello, when we decided to add templates we knew we needed to do it in a way that would be helpful for our users and also highlight the uniqueness of Trello use cases. That's why we decided to build a community-focused platform that would allow users to submit templates and highlight their stories in an easily digestible format. We wanted our templates to be less about highlighting our product and more about highlighting how the Trello community finds interesting, unique, and helpful ways to get things done - and share that back with the community at large.
Timing is also an essential consideration, part of why templates made sense to launch when they did was about what else we could build to serve our users' needs at the time. We bundled the gallery with the actual ability for users to create their own template boards internally for team usage, along with card templates, updates to automation, and more. The importance of timing cannot be overstated enough. Just because a feature isn't a priority at the moment doesn't mean it never will be - stay on it and collect evidence to build your case!