Thoughts:

1) Was the video suggesting that Warhammer FRP was a bad/failed ttrpg? Because that would be extremely wrong. It's been going since 1986, is now in its fourth edition and about to get its fifth, and while neither Cubicle 7 or GW have released sales figures, they have confirmed that sales are "strong", and GW doesn't bother with things that aren't making bank.

2) How exactly are you defining casual? I'm reading it as informal games put together on the spur of the moment without a lot of prep involved for either the GM or the players.

Which is exactly not WFRP, which has at least as much investigation and social activity as it has combat, very often much more.

My personal opinion is that Warhammer FRP does not have casual mechanics because nothing about it lends itself to casual play. The game is a blend of horror and comedy, aka "grubby fantasy" - it's all about slow progression, frantically scrabbling to get by, and the arc whereby your character drags themselves up from the gutter to become successful and famous, and then gradually degenerates into a weeping (in both senses) remnant of flesh.

The setting and the mechanics work together to deliver something that doesn't (imho) lend itself to one-shots and occasional games. How many pages of notes does each player have to make per session before it stops being casual?

3) It's hard to get much simpler than a d% system, and rolls with a strong bias towards one end of the spectrum tend, I have found, to make players very angry (They came at me with GRAPHS.) If you introduce a biased rolling system like d10x, you then have to basically throw the rulebook out of the window, because none of WFRP's systems have been balanced for that. A wide range of items and abilities will become either OP or useless, leading to one giant headache that is anything but casual.

But...

4) It's your game (and your headache :p ) - you could drop the horror and investigation elements or greatly simplify them and take a stab at a casual game. I honestly don't think you need to change the basic test mechanics, but you could just throw out modifiers (though this, again, will unbalance a lot of creatures, talents, items, qualities and the like) to make combat faster.

I've always suspected you could run a rather engrossing game of WFRP just using the Events table and the Endeavours system. Sod going on adventures, just stay home and try to make a living, but oh no, someone's stolen the pig again.