It's not necessary to always have your marksman bring nothing but DPS, if you have DPS + Utility in the marksman role and then put a hyper-carry in another role to complete the DPS in the team comp then you still have a balanced team comp.
So for example in the first team comp I gave it has Tristana which is a strong contested hyper-carry who can do very well in the mid lane, you have Nami + Kayle + Lulu + Nunu which is a a lot of buffs and protection for her or anyone that needs it. Also Kayle's late game is not weak at all. Lulu would be a weaker late game but makes up for it with utility as long as you bring damage from other sources. Let's say I swapped Kayle and Tristana in that team comp what does it look like then? It would look like a fairly standard protection team comp with a hyper-carry but just because I swapped Kayle to bot lane means it's suddenly not viable? Even though we've seen Tristana mid be viable already, and if you want to argue that Kayle's laning phase in the bot lane would be troublesome then you can opt to do a lane swap.
Viability isn't always determined by just how good a champion is by itself. The meta can also be flexible if you're willing to accept changes in it. Most aren't willing and so most don't try but even running double bruisers bot can still work if you can find a strategy that revolves around it but who wants to put in that time when we can just follow what other people have already found success with? This doesn't mean it's not possible or that it's not viable it means that no one wants to put the time or thought into a strategy that can be successful with something off-meta.
Viable literally just means that something is capable of working successfully, and if you want to say something is competitively viable then it just raises the level at which something is capable of working successfully. You say Lulu, Kayle, Teemo couldn't be played bot successfully because their DPS just isn't what it needs to be but instead you should look at what they do bring. Teemo brings late game map control, Kayle brings respectable late game DPS and a game changing ultimate, Lulu brings mid game strength and late game utility. With that in mind you build the rest of your team comp... so if it's damage you lack then put damage in your solo lanes like Ryze top with Jayce mid for an example.
Mid: 
Bot: 
Jungle: 
Mid: 
