Why would Barcelona buy him or more to the point why would Mahrez accept. His dream is to play for Barcelona, not go there and not play most likely. Neymar and Messi, which is he going to displace from the team exactly. He plays on the right so Messi, literally no chance. Neymar, more, he's apparently dragging on a new deal but that has a couple years to go. He'd be utterly mad to sign for Barcelona and basically sit on the bench for a few years. barcelona also have from my understanding a fairly worrying wage bill in terms of their income along with big stars who they are looking at new deals for which means more money.
Arsenal, again Leicester would be mental to accept any kind of exchange for Walcott, Walcott is utter crap and Arsenal would be asking him to leave a team that can win things for a team that can't.
Both Mahrez and Kante are young enough that they can stay another year and still get big deals anywhere else should Leicester fall apart. Should Leicester hit the knock out stages and do well in the league again there would be less reason to leave. Leicester will get over 90mil from league earnings/tv money this year and their wage bill was 48mil last year afaik. It will have gone up but not dramatically, that means there is a huge amount of money left in the budget to give Mahrez and Kante in particular fairly large deals to keep them around.
Winning the title with Leicester is a huge deal, it's history but 20 years from now people won't necessarily remember individual games, playing Utd or Chelsea isn't itself a historical thing. However next year Leicester might end up playing Barcelona in the knock out stages, that would be a potentially 1 in 50 year game, something the players and the fans would specifically remember basically forever. I think the players would be mad to leave when next season is the reward for winning the title this season. Sure they might suck next year, more games with a smaller squad, more difficult games overall, a few more injuries and more games could send them into Newcastle level freefall... so what. They might do a Spurs, have one more great season and get to a quarter or semi final. Kante and Mahrez can get loads more money from leicester next year then when Barca or whoever comes calling the year after they'd get even better deals, it's win win to stay.
Put it this way if Mahrez was on 40k a week today, then Arsenal would offer him 80k a week for next year and he'd be on that deal for 4 years. If he signs a 80k deal with Leicester then leaves the year after teams will offer him 120k instead and he'd have that deal for 4 years and he'd make more money overall.