The main issue is that it is ancient.

The cost, power, and length issues meant that it wasn't exactly well-received by the datacenter market back in 2006(!) when it was first released: DAC was the far more attractive option for a link from server to top-of-rack, and fiber was obviously superior (if not plain required) for anything beyond a hop to the next aisle over.

This left an incredibly tiny market, so obviously beyond the initial investment very little effort was put into developing new products for it. So now the prosumer market is hitting the limit of 1Gbps, 2.5GBASE-T and 5GBASE-T (both based on the techniques of 10GBASE-T, by the way) are becoming the norm, and suddenly network vendors remember that box of ancient 10GBASE-T transceiver chips that has been collecting dust in their warehouse.

Aaand suddenly you've got people buying what they think is a brand-new technology, but which is actually designed and manufactured using technology from a decade and a half earlier, and 10GBASE-T gets a bad name for being "hot" and "power-hungry". Turns out it is actually reasonably well-behaved if you actually make use of modern technology!

I expect we'll be using it for quite a while. 25GBASE-T and 40GBASE-T are even deader: A standard from 2016, which a decade later doesn't have a single available product? Mandatory switching to Cat8 cabling - and only a 30 meter range??? And no forwards-looking compatibility? Yeeaah, no thanks.

Right, but for example my fiber ONT only has an rj45 port so I'm stuck using one of these for the link to my UDM Pro. The router and my core switch use cheap DACs.