The bending over backwords mindset to appeal to casuals has created a more non-chalaunt mindset of players towards the game as a whole. People you straw man my argument though, hear me out. I want to start by pointing out that this isn't just a "casuals ruin shit, qq" nor is an attack on casual players. Furthermore, I'm not saying that any one reason is causing this.
Since Cataclysm, I have witnessed 13 raiding guilds collapse due to the snowball effect of people no-showing and taking raid times extremely lathargically (ie: showing up up to 40 minutes late, not showing up at all, etc; and not bothering to give a reason why or giving a very blatently poor reason or an obvious lie). And this is among heroic raiding guilds, 25man and 10 man alike. Casual players are becoming a larger portion of the gaming population, joining raiding groups and showing up when convenient to them (even the ones who look legitimate on guild apps and interviews). Hardcore players, over time, get burned out from the frusteration of dealing with this and start becoming flaky as well (if not randomly going on an unannounced hiatus altogether). This snowballs until a guild collapses, guild after guild.
With this trend, there wont be a point to adding heroic modes in future expansions/patches
I think you are comparing different things. People not showing up has more to do with lost interest than being casual. In fact if you were doing heroic modes in a raid ing guild, I would NOT call those people "casual". Those are serious raiders buddy, and the reason they don't show up probably has more to do with them being bored with the game than any kind of impact "casual" players might bring to it.