Business is often not so simple or easy to understand to those with less experience. I think he is saying that this deal is going to change the landscape of how sponsorships work and it may not be overnight, but the market will adapt.
I'll give you a quick ELI5 version...
Many companies thrive on the UFC. Meaning, if it weren't for the UFC, they wouldn't make much money. So they spend a certain percentage of their operating budget on endorsements to promote their brand. If they don't spend this money on promotion, they lose visibility and sales start to slump.
So let's say there are $10 million dollars spent on fighter endorsements each year. When these companies are banned from advertising during the event, they now have an extra $10 million dollars to spend on promotion. If they don't spend it, no return.
So where do they spend that money to get visibility? A larger portion is going to go toward fighters outside of fight week. Fighters will do signings, appearances, social media work, commercials, etc. So it's not that this money will just dry up and fighters won't make any of it.
... But they will likely make less of it. Some sponsors don't need the UFC. They will just more their marketing dollars elsewhere. How large this percentage is, is yet to be determined. But once we know what that percentage is, we will know how the market will regulate.
So let's say fighter endorsements drop by 25%. At that point, we will evaluate what 25% looks like. It will vary from fighter to fighter, but let's say for a particular fighter, 25%=$10k. So the fighter takes a $10k loss on each fight.
So then we evaluate how much the new Reebok money adds to the pot. If that fighter gets more than $10k from Reebok, then he comes out ahead. If he gets less, then he eats those losses.
Another significant factor, is that converting the market from fight week to non-fiction week promotions will increase the opportunities for fighters. Fighters spend about 3-4 months between fights. So that's 49 weeks a year that he can earn endorsement dollars. And remember, now the $10 million dollars is spent on these weeks doing promos outside of the UFC.
It may turn out good for the fighters. It may turn out bad. But what Dana is saying when he says "think long term", is to wait and see what happens. The current sponsors aren't going to just go quietly into the night. And they will quickly realize that sponsoring a UFC fighter outside of fight week is still more lucrative than sponsoring a One FC fighter during fight week.
If it turns out to be bad, then fighters should consider better opportunities. But making that decision before they know how this deal is going to shake out is a poor career move.