I think for a lot of owners it's not just about a production concern moving forward when a WR hits 27/28, it's about the value and tradeability of an asset. Even if MT keeps playing at this level, I doubt his value will keep increasing (or even stay the same arguably) because age has a big impact on market value, regardless of production.
I'd say the person getting Moore a) believes Moore in on a similar superstar track to MT, b) in the meantime can give a large % of MT's production, and c) will have higher trade value (or more accurately will be coveted by more owners in the league) over the next 2-3 years. Put simply, Moore gives you the upside lottery ticket, a high pedigree and profile and value insulation. Of course this all assumes that Moore becomes a perennial top 5-10 WR.
In terms of getting more for Thomas right now, maybe in theory yes, but honestly I think this is about the most you can get if you are trading for WR - an elite young prospect who has already produced to some degree and a first to cover the gap/risk. Once you start putting more assets on the Moore side, the Moore owner probably starts to think what the upside in doing this trade, especially if you're paying for past production to some extent? Not saying this is right, but that's the thinking. Also depends on how your league values firsts - I've been in some leagues where they are almost used as throw ins to even up a deal, and others where they are highly overvalued.
The other consideration with MT is the Brees situation. Now DJ has an uncertain QB future too, but what if MT has Tayson Hill at QB next year and the Saints move to more of a Ravens type offense. That could happen. Maybe the MT wants to combat that risk and getting a coveted asset like Moore and a first on top isn't a bad way to do that. Cashing out for youth also makes a bit more sense in a vacuum this year because of the uncertainty surrounding this season.