A lot of steel Merckx bikes are popular, at least the 2nd generation ones. The 1st generation (Falcon) Merckx bikes don't get a lot of love.

However, only a few brands really are holding their own in the vintage market.

Colnago.
Merckx?
Sometimes Bianchi
Older Paramounts

Others can be a bit hit and miss. Merckx will likely slowly fade into history, while companies like Colnago will continue to sponsor racing teams and innovate on the cutting edge.

Did Merckx do aluminum frames? Carbon forks?