The real question is "what if there are several accounts used to bump the thread?" i.e for promotional purposes.
That won't last long. Remember the new bump calculation and you'll realize that it takes more than just multiple accounts spamming the thread or clicking the 'bump' button. Even if you have 10 Legendary accounts, your thread won't stay on top forever.
Yes, i realize that a single person cannot bump the thread to remain it on the top. I think for people the target now is not to be at No. 1 position, but at least on the No. 1 page of the Section and this can be achieved to some extent by (as you mentioned) by few Legendary accounts or by paid bumping service.