See that doesn't make sense to me. What's the difference between top and bottom lane? Sure I get the jungle because the mobs in there, but from what I can tell top and bottom are extremely similar. I feel like if it's a general rule where every league player uses specific champions for specific lanes, then there's so much possibility for a counter strategy that would just dominate the map.
About top and bot differences, it is mainly about this
Since dragon is quite relevant for the winning of the game (both before and after the changes), mid and bot lane become quite important: if you can find a moment when enemy team isn't around, you'd want the most people available to help clearing the camp in the fastest way possible, so that enemy team can't steal it
To do this, meta changed in this way
----On mid, you usually find mages and assassins: since it is a solo lane (it's the smallest one), people usually get champions who deal high amount of burst dmg so that they can easily shut down their opponent in lane
----Bot is the largest lane closer to the dragon pit, thus you usually put here 2 champs. Since the other team puts another 2 champs in it, you'd want to get a champion that deals costant high amount of dmg (a marksman) and someone who can protect/help/assist him (a support). In this way, your marksman can not only quickly help with dragon, but also has someone who can cover his weaknesses (and since he must "carry" the game, it is quite important)
(Btw, since the marksman deals mainly AD damage, mid players usually pick a mage or an AP assassin so that enemy can't prioritize only one kind of defense)
----Now, abot top: it is the farthest lane from where usually the action take place (dragon pit), thus you can almost consider it as a place that is usually forgotten by everyone (except when a tower is lost). Because of this, people usually get for top lane fighter champs, whose abilities make them good in 1 vs 1 situations. However, since it is considered a "lone lane" almost everything can go there. Usually people take this in consideration when choosing a champ for it:
-an ability to sustain yourself during lane phase (Garen, Vladimir, Maokai, ecc.)
-the possibility to deal high amount of dmg, be it burst (Kennen, Lee Sin, Nidalee, ecc.) or per second (Tryndamere, Irelia, Jax, Rumble, ecc.)
-good crowd control abilities and tankyness that helps you winning mid-late game teamfights (Maplhite, Shen, ecc.)
-godly split-pushing abilities (Nasus, Singed, ecc.)
However not always they abide for it: just to say, sometimes I meet assassins and marksmen too on top
About your second sentence, your thoughts about it are quite correct, in the sense that people get fixated with the current meta
Actually, there are already some things that we could say "break" the meta (and not in the shitty ways some youtubers (or most of them (fucking keyori)) show)
Examples:
-I don't know if it is true, but it is said that a counter for marksman + support botlane is going double bruisers (fighters) bot
-Having a mage as an offensive (Viktor, Xerath, ecc.) or defensive (Morgana, ecc.) support can give your team another AP carry that can deal lots of dmg in fights
-Twisted Fate jungler is OP: by having only as offensive items boots (don't remember which ones, but I think atk speed ones), Stalker's Blade with Devourer and Wit's End and going full tank for the others, he was capable in one match against my team to completely obliterate carries and melt tanks while being as much resiliant as a tank (a nightmare)
-Going double top with a fighter and a support can usually be better than having someone as a jungler (supposed you finish top lane quickly (leaving your opponent incredibly behind) and your teammates can play without jungler support)
-Going bruiser mid can counter some mages or assassins, since you're too resiliant to be bursted down and they're too squishy to withstand your dmg output
Mainly, though, it depends on how the game goes (obviously)