Well first and foremost, we're conditioned to expect that. Why? Because in the last decade we've seen 2 expensive DHs on this very team basically do that to the letter: Dunn put up one of the worst seasons in MLB history and LaRoche was so bad we're glad he retired. Both of them produced better than Abreu did this season and then fell apart the next year. Worse still, fastball velocities have gone up league wide, especially since the Dunn signing, so if the thing that causes guys to fall off a cliff is that they slow down too much with age, the risk is still quite high that this player will hit a cliff at some point. Given our recent history with the DH position it'd be more surprising to us if he didn't fall apart than if he did.
Secondly...we already have seen substantial declines from Abreu. You like batting average, his batting average has dropped a lot. His strikeout rate has gone up. He's swinging hard to still hit home runs, but if that last skill starts to weaken he has nothing left. So already he's not nearly the hitter he was 3 years ago and we shouldn't pay him as though he is.
Third, you haven't bothered to understand any of people's actual discussion of Abreu, which isn't exactly surprising considering you don't bother to understand most things people write to you. While he may not fall off a cliff next year, or the year after, it is also not unreasonable to expect his performance to continue to take steps downwards, as we've seen this season. That may not happen, he may have a career year left in him, but people don't want to pay him with the expectation he'll have a career year and you don't want to lock yourself into him if he does continue the path downwards he's already ono. If we had a great lineup in 2021 and Abreu took a major step back that year, playing him every day at DH because he's well paid could be the thing that costs us the division.
So, just about everyone is ok with bringing him back for 1 year at the right price. A number of us are ok with 2 years or 1 year and an option for a 2nd year. Almost everyone who looks at this says that if he wants anything beyond that, he's going to have to hit the next 2 years because no team in baseball would sign him to a 3 year deal when his performance has already dropped off a lot. Also a lot of us understand that his free agent value right now is something like $10-$12 million on a 1 year deal, so anything beyond that is over-paying for a guy, and under Reinsdorf's roster, overpaying 1 guy means we have other guys we can't sign.