The Empire of WHFB could definitely not be compared to the Imperium of WH40k in regards to popularity if you include Space Marines in the equation. The Empire was overall one of the more/most popular factions in WHFB, but probably at no point the most popular one.
If AoS is anything to go by, "ordinary" humans are apparently almost completely out of fashion these days. Bigger, better, extraordinary and magical is the name of the game. That's also why I do not have high hopes for the reimagined Warhammer Old World.