I imagine it has definitely been thought of.
At this point, however, it's difficult due to a few issues.
#PRICE
How do you suitable price all content (or perhaps all FUTURE content as well, which is the appealing purchase)? As of 2014 League of Legends had something like $2,615.00 worth of content at the most efficient RP price point. It's gone up since then, and goes up every year. If you DO find a price point and it's lower than $1,500-$2,000, some players will be (very rightly) annoyed at spending more than that before the option was available...and refunding them isn't financially viable, since the required high price point will make this an uncommonly purchased item.
#FTP FINANCIAL PATTERNS
In the industry there are typically four basic patterns of player: free players who spend next to nothing (maybe buying 1-2 champions or skins at the most), occasional spenders who buy occasionally and don't spend much money at any one time, content collectors who have a few categories they'll purchase most things within, and whales (I believe that's the greater industry term) who spend VERY large amounts of money (a content collector who wants ALL the content is one of these as well).
As a result, a lot of money is focused in those last two categories. You make a solid amount from the millions of occasional spenders, but putting a hard cap on how much people can spend on your game is, unfortunately, cutting away from the purchasing potential of your top spenders. In a game that survives on microtransactions, it's sadly not the most practical idea to put an upper limit on things -- the people who would buy an "All Content" access for a very high price are usually going to be those who would end up spending that much or more anyway.
Unfortunately, that's the harsh reality of the microtransaction model, and it's not just something Riot does: it's a pretty consistent pattern across the industry, and part of the economic structure of micro-transaction based gaming. People are (oddly) more likely to pay $100+ in small increments over several years for a game that they wouldn't buy outright for $60 -- look at Clash of Clans and Candy Crush Saga as examples.