Uh, yeah. I know people who spent nearly 20 years pushing to build a single trail (or open access to an existing one). Ebikers do need to understand that we've had it really good in how rapidly we have gained access and to how much. It isn't perfect, but its gone (and continues to go) really well.
Sure, all groups are open to more members, and all groups work to get and maintain trails. I disagree with your assertion that there is a widespread "drawbridge mentality". IME you have the most success with advocacy when you build relationships with those existing groups. Their concerns (that these trails were built with non powered use in mind and we would like to bring powered vehicles onto them) are not irrational. Most of them have zero experience with ebikes. If you come in and demand access and try to force it, it isn't going to go well. As I said, a lot of trails were built and are maintained by those groups. Probably 80% of the mtb trails I ride were advocated for, built by and are maintained by the local IMBA affiliate. They do several thousand man-hours of trailbuilding and maintenance per year and have for decades now, and they have tremendous trailbuilding knowledge. That gives them clout. Meeting up with leadership members of that org (I'm friends with a few of them) and letting them ride my ebike is worth more than ten letters to the CPwhatever or a million words typed onto a forum.
As JR has said a few times, land managers are not the enemy. They generally are really passionate about the trails they manage and want to see them being used by the widest range of people possible. Their concerns about ebikes are legitimate. Doesn't mean I think ebikes shouldn't be allowed access, I obviously do. But the process isn't to try a top down forcing of the issue, its to do it the way everyone else does. Build the relationship and demonstrate that we can be good trail users and stewards.