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.
Being a legendary member doesn't mean you can bump a thread in those four targeted boards.
If you are a legendary member, but have not earned any merit in the last year, you cannot bump any thread. There are some legendary members in the forum without any earned merit. Their bump power is zero.
What determines your bumping power is not your rank. Your bumping power depends on merit you have earned in the last year and your activity assuming you have signed up one year ago.
So, a junior member can be more effective than a legendary member.