Possibly but that's why I think it critical that you get into it on merit through the domestic league.
If you have United or Liverpool guaranteed a spot in the Euro League every season, then yes, they will probably play weaker XIs in the Prem. If they had to qualify for the Euro League every season, like they do now with the CL, they couldn't afford to take the Prem lightly for fear of not qualifying.
Plus, lets see how much these teams enjoy finishing mid-table in a Euro League every season and throwing away the Prem at the same time. These clubs are historically used to winning trophies, as are we in our recent history. Fans are not going to tolerate throwing the domestic competition, so the clubs can line their pockets every season, whilst potentially never winning the Euro League because the competition is so strong. Can you imagine United or Liverpool becoming the Euro League equivalent of West Ham (No disrespect to WH)?