And *why* is it a bad idea to send AD carries mid, or send 1 champ bot, or not have a jungle?
All of those are influenced by mechanics of the game. Unless Riot actively changes the way the game works (Let's make it random whether dragon and baron spawn top or bot!), there's never going to be a massive change in the meta.
Can anyone tell me why anyone WANTS to change the meta? Why? Who cares? The meta is like that BECAUSE it's been shown to be effective.
These are really easy to answer questions. Sending an AD carry mid isn't necessarily a bad thing. It's just that many AP champs can throw an AD champ back to base. You end up playing passively, which again isn't necessarily a bad thing, but an aggressive mid will make you lose gold early, which you want. So they get stronger and stronger, and you only get just a tad better.
Now, if you manage to have an AD mid, then you've got a few choices bot. You can go double bruiser/tank bot. That is good if you manage to be huge bullies/they have weak sustain. You can also do AP and support/tank, which isn't nearly as great, since you lose out on levels. You can also do another AD bot with support/tank. This means double AD carry, which causes a few problems. You'll have more armor being built to mitigate two players on your team, meaning you'll either need an AP jungler (doesn't scale as well) or an AP top (does a pretty good job). If the AP top gets destroyed early, your team is not going to put out enough damage.
Now, let's look at the best choice, double bruiser bot. For this, you'll have an AP top, AD mid, and double bruiser bot with a wild card for jungler. Your bot lane won't be nearly as tanky as the other team, but there will be at least two of you manning the front lines, so that helps. However, unless you have a support jungle (actually pretty doable), you'll be down someone to support, which seems like a useless teammate, but basically can make or break a game.
As for sending one champ bottom, this is actually an occurrence that happens. You can send someone with either amazing sustain, long range, or incredible tankiness to fight two people bot. You only want to do this for a few reasons though: 1) If your solo does well with little cs, 2) if their top is someone that is very aggressive, or 3) if they have a passive lane bottom.
By sending one champion bottom, you lose dragon ward coverage, which is the main reason two people are sent bottom (instead of duo-lane tops). That means dragon gets hard to contest, and could subtly lose you the game.
For not having a jungle, that usually loses you the game. Having a jungler allows for ganks, which can literally win you the game. If you choose to do duo-lane top or mid instead, you get to dominate one lane in exchange for no jungle control, ganks against you, and overall stronger opponents.
Duo-lane mid would actually be the best choice, since you are free to gank top or bot from that lane. However, you will probably be outleveled by their jungler and mid, meaning that they will be able to hold that lane against you eventually. Your only real hope is to either crush their mid early on, invade the jungle, or gank top and bot a lot.
Basically, it just takes more effort to duo-mid than to have a jungler, who would do all of these things anyway. The only upside is the dominated mid, meaning he will be playing pretty passively.
While that's true in general, you have to factor in lane matchups. Some ADs can utterly demolish many APs. Sivir vs Annie, and anything vs Kassadin are brutal for example. Then you put your AP top and run double tank bottom to potentially deny the enemy AD. Or you just go standard for bot lane with a tanky support and jungler and have double AD.
And ninja'd. Yes, some APs are really hurt by ADs. All of the anti-mages REALLY suffer from ADs. Kassadin and Morgana are two that I can think of off the bat that would much rather fight an AP than an AD. Sivir laughs at many APs. But, usually, APs can stomp on an AD.