It is reasonable to think about future optionality, but I would not join a boutique that you do not actually want to work at just because you hope it leads to MBB later. There is no guaranteed conversion path.

A better test is whether the boutique is a good next step even if MBB never happens. If the work is interesting, the learning is strong, and the role gives you better experience than your alternatives, then the future MBB option becomes a useful upside rather than the entire thesis.

During the role, focus on building transferable evidence: client impact, analytical ownership, leadership, and strong recommendations. Those are the things that will make the experience valuable later.

So yes, it can be a stepping stone. Just make sure it is also a solid place to stand.

That is how I would think about it too. Good job first, possible MBB path second.

If you hate the boutique from day 1 because you are only waiting for MBB, that sounds miserable.