Pentangeli could/did have the impression that he might not like the Rosatos but "the boss" was making him play nice. That's way more believable given the circumstances and probably would have worked. But I do agree that Pentangeli was expendable.

However I believe Michael was not expecting Frankie to come to any harm other than the humiliating back down, perhaps handing over the three territories and in the process looking weak etc.

In my opinion if Frankie is killed and Rosato brothers, Roth's ally take over the Corleone operations then Michael loses his muscle and becomes just another casino operator, easy pickings for anyone which would be bigger worry for Michael?

Whilst “No one was indispensable” Michael would not sacrifice any of his men [especially his top men like Neri or Rocco] – [or his muscle Pentangeli] - whom Michael selects and counts on them to carry out their jobs successfully and get away with it

Every member is pivotal link in the chain
If lost as unavoidable collateral casualty, so be it but not intentionally "expendable" nor would Michael want to lose / sacrifice his men