There is a lot that can be addressed in your comment lol. Is this a cry of intent for help?
I'm going to assume call for help.
I think you're building your case and experience on a fundamentally flawed perspective. Yes, trolls exist. But I can't say that the majority of people queue up with the intent of letting people down for the joy of it lol.
Fact is your teammates are trying, in every elo, most ppl are trying. Going in with this toxic mindset, and beating the dead horse of not being able to rely on teammates doesn't do anybody any good.
My best advice, if you want to talk initiation/tanks, is to put more thought into the setup and the aftermath of your initiations. This will put into perspective whether your intiiation was truly good for the team, or bad for the team. Pay close attention, I said good/bad, not the theoretical optimal play. Depending on your elo, an optimal play can be TYPICALLY optimizing splash damage. As elo's go higher, an initiation on your enemy team's winning condition, a fed target, or a very disruptive support, at the right time has greater impact than a 5 man ult. Why? Because, your team was in a BETTER POSITION to follow up as people on both teams were en-route to a grouping point than they were in a mirror 5v5 ARAM under enemy tower.
That's just one probable outcome of many...but honestly, I think what's prob going to help you more is reframing your perspective. How can you best enable your team? Otherwise, play more splitpush/assassin/carry roles, or learn to spec your tank properly for that purpose. For example, a 5 man tank malphite ult is great because it does decent splash dmg(early, less late) but it also shreds armor, and hurts opposing team's damage capability by impairing movement and attack speed. You hit the team with a good knock up, slow everybody and just continue to deal sustained noodle damage and maybe run the carries out of the fight while your teammates melt the frontline. But...if you are ahead, and you THINK that you can handle the responsibility of carrying the team, you may consider by offensive items. Smaller life bars after your ult+rotation may entice your team better for follow up, ya know what I mean? Maybe blow up 2-3 people in the process and die or Zhonyas. You probably won't be running at their carries if they weren't caught in your rotation tho, different than tank Malphite. Maybe if all the gold is on the enemy Vel'Koz, it's worth trading your life for their team's only waveclear and threat at Baron/Dragon contests. In doing so, you'd have to play differently and realize that certain match ups that were traditionally favorable may no longer be favorable. You may now be a teamfight god, but if you walk back into your lane vs. generic AD Tryndamere depending on build/stage of the game, you're not going to have a good time.
TL:DR, look to enable your team. Or learn to carry properly.