If such a thing is believed by some, then Buddha remaining in the world being a belief that is similarly believed is open game how I see it.
What I believe is, after the Buddha's parinirvana, it's the Dharmakaya, the Dharma-body of all buddhas, which remains active in the world:
The Buddha is equated with the Dhamma:
... and the Buddha comforts him, "Enough, Vakkali. Why do you want to see this filthy body? Whoever sees the Dhamma sees me; whoever sees me sees the Dhamma."
When the Lotus Sutra refers to the Eternal Buddha, and says that all other buddhas are manifestations of him, I believe that's referring to the Dharmakaya.
Well, one can believe that, and even do it while calling themselves a Theravāda Buddhist, even more controversially, one can believe that and do it while being
a Theravāda Buddhist technically (like my partner's hairdresser, who I am sure believed
that thing about reincarnation to be "orthodox" Theravāda Dhamma, out of sheer lack of knowledge, not being especially religious but maintaining a devotional piety), as can anyone else, but you also have to be prepared for this:
David N. Snyder wrote:
Open game for shooting that view down?
Because, at the end of the day, there is
a reason why some things get called "folk Buddhism" within a tradition, for the simple reason that many people hold incoherent views or contradictory views regarding their tradition, having never actually looked at it critically, having learned the fundamentals as an infant and having lost interest afterwards. When this stereotype gets extended, it becomes a problem, but anyone can see that there is a certain amount of truth to the fact that this occasionally happens.