And here we go again. Okay, let's do this one point at a time:
I don't like their designs (Mega Ampharos being an exception). Most of them seem like overly detailed copies of the original Pokemon with spikes sticking out everywhere.
The thing is, some of the designs are a bit silly (Mega NicCagechomp springs to mind!), but the whole point of Mega Evolutions are that they're an iteration on a Pokémon's final form - they're Super Saiyan Pokémon, not entirely new creatures. So yes, it's often shown by adding details to a Pokémon - like spikes, or DAT HAIR, or Mega Absol's "wings". Completely changing the creature in question is not for Mega Evolution, it's a regular evolution.
Which brings me neatly onto your next point:
Will there no longer be any normal evolutions for Pokémon from past generations?
Yes. There will be. Please repeat this mantra a few times and learn it: "Mega Evolutions are not normal evolutions and will not replace anything." Game Freak decided to add new lines of creatures in Gen V and VI rather than augmenting the old ones with new evolutionary branches - in Gen V it was so they could build an entirely new Pokédex in Unova, and in Gen VI it was so they could focus on the new creatures and Mega Evolutions. If they want to add evolutions in the future, they will. They're not going to suddenly say NOPE, NO MORE IDEAS, WE'RE NOT DOING IT.
If you've ever read any interviews with anyone from Game Freak, the mantra is always "Ideas, lots of ideas, we like ideas, we just won't use them all in one game." There will be lots of evolution ideas lying around, and always will be as their designers think of them - just because they didn't use them in this game doesn't mean they're not going to in future.
Incidentally, this kind of jumping to conclusions is a real sore spot for me - One example of someone not doing something never instantly means that they've mandated that it won't happen again. The concept of Nintendo or Game Freak or TPCi or whoever suddenly deciding not to do something like adding new evolutions to prior lines, or ny similar ideas, is petulant and purely a knee-jerk reaction.
Why are most of the Pokémon with mega evolutions Pokémon that already have enough popularity and/or strength (Mawile and Banette being exceptions for this one)?
Popularity. Game Freak made Mega Lucario and the Mega Charizards and the Mega Mewtwos simply because they're fan favourites and they were bound to be popular - or at least, if they didn't exist Game Freak would be hounded by the fans to add them. But frankly I think out of all of the Megas, only a handful are insanely popular already. Let's face it, no-one was clamouring for Mega Abomasnow, or Mega Pinsir, or similar. A good proportion of the Mega Evolutions were to upgrade certain old (and, let's face it, not completely popular) Pokémon to be viable again - Mawile, Absol, Ampharos, Houndoom, Kangaskhan, Pinsir, Heracross, etcetera. Some of them were Megaed not to lose their viability with the power creep Megas brought - Garchomp, Tyranitar, Aggron, Scizor, etcetera. And some of them are because
DISREGARD THE CONSTABULARY, like Mega Gyarados.
So the rationale for giving Pokémon mega evolutions is not exclusively because they're already popular or strong. After all, Volcarona didn't get a Mega Evolution and it is the destroyer of worlds.
It's still the destroyer of worlds.
Both Mega Mewtwo X and Y are the strongest Pokémon in the game. 780 BST? Seriously?
Yeah, seriously. They're still not broken, though, because base stats are only a tiny piece of the puzzle. And you know full well that if the Holy Goat, Master of 'Ere He Trotted got a Mega Evolution, it'd have 820 BST or something. Mega Mewtwo have the highest base stat totals because they're the only (currently revealed) Mega legendary Pokémon - they already had 680 BST before they Mega Evolved, after all.