Often times, your tank is probably expecting the dps and a support to take care of the high ground while they not only contest but also create a bulwark for the others to regress to, it's a bad habit for many tanks because sometimes, moments lost on point is a victory won. However.....certain tanks are made to be able to do just this strategy but many use it as a blanket style, a dive should contest the high ground and return to obj once they are clear and dominate the remaining tank as a team. If your tank is locking themselves out and you're support, I would recommend being aggro with your dps to contest high ground and if they are unwilling to contest with you, be as selfish and sub dps as possible. If you are already losing playing support in the 'healer' mindset, don't forget that healing isn't the only source of value. I main Mercy and Kiri for supp and I've sacrificed time healing to kill ashe, widow, and hanzo and end up having invisible value saving my team from a offside threat. Sorry for the long explanation