Hi

As a piping stress engineer, I like the sentence "all thing are springs" which is from this forum and the thing is the spring stiffnesses are different. As you said "rigid" which stiffness could be defult, 1e12 in CII or you can define the number you want. In a system, the relative stiffnesses are important.

As you said normally the assumption of rigid supports (stiffness 1e12) is conservative, that means that the rigid point stresses could be conservative, but not all places as the rigid supports take more loads so the other place take less loads in the calculation.

This is only my opnions and I think you can find more discussions regarding this topic in the forum. Most time this point need to be judged based on diffrent systems.

Thanks,
Carter