I'd say a couple of things.
1) No-one has ever said it's impossible to compete with those sides but it is very difficult and you need a fair slice of luck. Liverpool have done really well this season and I hope they win the league, it will be good to see a club without the infinite money cheat do it, but where would they be without Suarez? They've had a bit of luck with him not getting any injuries this season. That's the sort of luck Chelsea and City don't need. They can have a top player injured and just bring another £40m signing off the bench. We can't do that. Nor can Liverpool. They've done really well this year but with the added pressure of CL football next year Liverpool's squad will be put under far more strain. Which leads me to my second point.
2) Will Liverpool sustain this level? They've been 'up there' before, but not consistently. What if City or Chelsea decide they're going to buy Suarez next year? Liverpool may have turned up their noses at our £40m (and one!) bid but City could offer £80m to Liverpool and £300k a week to Suarez. They can make an offer which Liverpool would find very difficult to turn down, especially if the player's head is turned by a huge pay offer. And as Spurs found this year, buying a load of decent players to make up for the loss of one outstanding one doesn't necessarily make you a better team overall. The losses Liverpool have made over the last 2 years are not sustainable so how are they going to build on their success this year? We have already been through the pain of a stadium move and with the new commercial deals are well set up long term to compete. Last summer we saw the first signs of more ambition in the transfer market, I'm hoping that will continue this summer. Liverpool have been talking about a stadium move for years and not got anywhere much, unless they manage to do that it will be impossible for them to generate the kind of revenue they need to compete with City or Chelsea long term and if they do move they'll have a huge debt to repay which will make competing even more difficult.