That's a good question.
Given that Sony is still charging for their first party titles and they continue to sell well, I bet they feel less pressure to raise prices on PS+. Any increase would hopefully come with some additional benefits and should be modest by comparison.
Every company will go after more money, if they think it's valuable and achievable. What I mean by that is nobody will increase the price of a services, if they expect many to cancel their subscription. This could potentially end up in a loss.
However Sony has shown in the recent past with the price increase for games that yes, they'll absolutely increase the price of something, even if they could've continued selling it at a lower price. Their games are extremely profitable as it is and there was no pressure at all. Neither Sony or Microsoft are consumers friends. They'll do whatever is necessary to get more profit.
I dislike the decision to increase the price, especially with Gold. The offered games with the exception of this upcoming month were really bad in the collection and while I actually do think XBL is the most modern and stable onlineservice, it doesn't warrant such a huge price spike. Poor decision imo.