I think if he easily destroys Spence then he may fight on, but if he has a real tough fight then he'll retire.
Might depend on how he wins. If he dismantles Spence then maybe he fights on thinking he can keep winning but if it's a real tough hard fought victory maybe he sees it as the perfect swan song. But I suspect he'll fight on, he's a born fighter, loves to fight more than even most fighters, I think it's going to take a beating from someone to make him retire.
If Pac were to beat Spence, I hope he does retire. This fight is going to take its toll on Pac. Whether hardcore Pac fans want to admit it or not, he looked old and slow in the latter part of the fight against Thurman. He was getting worn down and it clearly showed. However, that could be because Thurman is a strong puncher, so that could have played a major role in their fight. Anyhow, I hope Pac has one more great fight left in him and he retires in good health. It would be a shame to see him be used as a stepping stone for these young, hungry lions. On another note, I would have favored his chances more if he were to have fought Crawford instead. Crawford isn't as long as Spence is and his size and reach wouldn't be such a handicap. I'm rooting for Pac but wouldn't be surprised to see Spence TKO him in the later rounds.