I agree, but the argument here is that you make this kind of trade for exactly that reason.
Kyrie Irving, as he has existed in the league to this point, will contribute less to the Boston Celtics than the combination of Thomas and Crowder (disregarding the pick and Zizic). And I think those guys will make the Cavs better than they were. So if you are excited about this trade, it's because you are a believer that regular season is simply irrelevant. Or you think Thomas is going to be a very low value player because of his injury and that the Celtics wouldn't be able to get by without a premium PG. While the latter point is fine, reading most comments it seems the overwhelming feeling of supporters is that you do this trade to have the best players on the floor in the playoffs. So the question of where Kyrie Irving fits on that ladder is an open and valid question, and the answer to that question seems to be the fulcrum of opinion on this trade.
Putting this another way. Before the trade the Celtics were a 55ish win team with a good chance at the conference Finals and iffy chance to make the NBA Finals. The only way to improve on that meaningfully is to improve your chances in the playoffs. Hence, the team's playoff performance with Kyrie is essentially the only thing that matters to evaluating the trade.
But the trade is more than just about the 2017-18 Celtics.
We have likely seen the peak performance years of both Isaiah and Crowder, with the former being subject to a cliff-like falloff, assuming he'd even be on the team after this coming season (an increasingly unlikely scenario). So, maybe Cleveland gets a little bit better next season, but between the likely decline of IT and Crowder, and the possible departure of LBJ, it seems likely the Cleveland's window ends after the coming season. And, in 2018-19, a team with Kyrie is almost certainly better than a team with Crowder and IT's empty roster spot.
The Celtics weren't going to be players in the 2018 free agent market, regardless of this trade. So, Ainge's choices for the future team were to:
a.) Hope that the combination of the BKN 2018 pick, the other future picks (LA/SAC/PHI pick the most prominent), and the newly drafted players can elevate the team organically.
b.) Hold on to the BKN 2018 pick for a future trade possibility.
c.) Trade the pick for the best available player now, which happens to be Kyrie. (key word is available).
None of the above are certainties in terms of outcome. But Kyrie is a very good player who's 25 years old. And, if he doesn't work out, he could very well have trade value next offseason.
As for next season's playoffs, I'm not convinced that Irving would contribute less than the combination of IT and Crowder. Jae would be more of a rotation player, and who really knows how well IT's hip holds up during the course of an endless regular season and a grueling playoff round or two. The one thing we all loved about Isaiah was that he was absolutely fearless and gave it his all every single game. That style of play is not conducive to longevity, especially when dealing with a hip injury.