Are you kidding? System is in its Prime.
First off, the prices. I could sit around all day and talk about TLG's "Ridiculous Prices", but this article from Reality Prose covers it rather well.
Now we have the minifigures. Look, I understand that some fans want realistic figures in their sets and want LEGO to change up their figs, but they just can't. They would be destroying a nearly forty year old pop culture iconic. That's like Disney scrapping Mickey Mouse. If you really want the more realistic and, in my opinion, less customizable minifigures, I would go to TLG's competitors.
I'm not a big fan of the realism argument when it comes to why Mega Bloks are better. LEGO has shown that they can make great looking, realistic-looking models. The Mini Cooper and Volkswagen Camper Van re probably the best examples, closely resembling the original vehicles. Besides, LEGO subjectively has better builds.
For my comparison, I'm using the 213 piece set, Halo NMPD Warthhog,
And the 220 piece LEGO set, Marvel Superheroes Avengers Hydra Showdown.
These two sets are of similar size and includes at least one four-wheel vehicle from a licensed theme.
The Difference is rather apparent in my opinion, with the Warthog using quite a few pieces that leave very, very limited use anywhere besides this set, such as the cage looking thing on the back. The LEGO set's black vehicle however uses less far less of these unneeded and useless bricks, instead using pre-existing bricks to create a semi-realistic vehicle, albeit not as realistic as MB's (But that's mainly from Mega Bloks' Color Palette.)
In the end, it all comes down to your personal preference. Do you want realistic, yet limited products from Mega Bloks or do you want the "cartoony" but complex sets of LEGO?