You cannot attract players to tanking and healing by making their gameplay more forgiving and more accessible. It just ends up devaluing the importance of these roles.

Survivability in this game is secondary to DPS. Individual deaths really only matter because of the impact that they have on dps. You're probably much more likely to wipe if a dps dies than if a tank or healer does. Raid importance gets reflected in gearing priority. The less value you bring, the later you get geared. And with a widening dps gap between 'true' damage dealers and the other roles, it's not particularly difficult to see why there's less value in playing them.

Historically, this was offset by the fact that good tanks and healers could put out some fairly competitive dps. If you don't want them to be able to do this, you have to give them some other means to provide value. There needs to be more to survival than learning team jump rope mechanics. Deaths, especially those involving tanks and healers, should not be recoverable. Damage, especially on your tanks, needs to be convincingly threatening. Untanked mobs should chew through your team in seconds. There needs to be a raidwide limit on raises per fight. Your tanks and healers should be preferentially the best geared players in your team because you need them playing optimally in order to survive the fight, and you should want the very best players in these roles to shepherd you through safely. That's how you get people to play them. Make them feel needed.

DPS is where the sub money is at. That's why we have multiple subcategories of dps that are each individually considered to be equal in importance for new job development to tanks and healers. But people migrate to roles that carry value to their teammates. If you want a trinity, you need to increase the value of the other two roles, by changing gameplay and job design to reflect this. Failing that, at least let us all at least be pretend dps.