Netherlands attacker Robben, 29, France winger Ribery, 30, and Germany midfielder Schweinsteiger, 29, were all key players in the Bavarians' treble success last season and remain an integral part of Pep Guardiola's team this term.
Nevertheless, Lehmann believes they will not be able to maintain their current level for much longer and has advised the Bavarians to start looking for replacements.
"Bayern will not continue to win the Bundesliga year after year because this team will be at its peak in one or two years," Lehmann told Sport Bild. "They have to replace players like Robben, Schweinsteiger and Ribery at that point. Schalke and Dortmund will have to be ready when that happens to make the Bundesliga interesting again.
"You never know how it wil pan out when you replace seasoned stars. There are plenty of examples of where it went wrong."
Robben has a contract with Bayern until 2015, while Schweinsteiger's current deal runs for one more season and Ribery's until 2017.