Because people are buying more now I would wait. I remember Yamega Prime being a $40 card when it was in standard but now you can pick them up pretty cheap.
They most definitely go down over time, just look at roaring skies shaymin for a recent example of a highly played card that rotated out of standard.
Cards are always expensive when a sets released, they get cheaper as more product is opened, the cards WILL go down because people will keep cracking boxes to sell singles if the box EV warrants it. Some chase cards will carry the box EV due to being playable, they will drop once they’re not playable like shaymin has.
With new sets playability is always a factor in card prices. Once a set rotates out of standard the set is still in the expanded format for a while after that which again is a factor to a lesser extent than cards in standard. When the set goes out of print this stops the cards being pulled so frequently and slows the new supply hitting the market. Basically once the players stop buying (demand) and the set goes out of print (supply) the price of playable cards will start to recover from the low it hits when they rotate out of standard and the “collector” value starts to come into play which is where they will settle. Remember that we are in unknown territory in with hyper rares at the moment as no sets have had this tier of rarity before sun and moon so it’s hard to say how they will perform in the future. Print numbers are also very high at the moment so who knows they may just be readily available cheap(ish) cards by the time they are out of print.