Other posters are giving you flak because you're not clear on the word 'rate.'

All rates can go up or down, but a rate is not any old thing that can go up or down. The balance in my bank account is not a rate, it is a total. Rates are measures of frequency.

WAR per game, WAR per PA, WAR per 650 PAs—these are rates. WAR is a total.

There is a difference between a tallied stat like home runs and a number like WAR, which can be lost. But the difference isn't actually that interesting here, and it's more similar to your home run example than you think:

If Trout has a poor second half of the year, he's not going to live up to the pace he has established. Same exact thing with home runs: you can be on pace for 70 but wind up with 45.

But for his WAR to actually go down over the course of the rest of the season, he wouldn't just have to slump, he'd have to play out the rest of the year like he didn't even belong on the bench, he belonged in the minors.

Theoretically possible, but not really worth considering, anymore than it's worth considering the theoretical playoff hopes of a team 20 games out with 30 games to go in the season.