You are proposing an interesting thought experiment or hypothetical (“to what extent was the design team expecting a 10x longer lifetime?”). That’s worth pondering.

Furthermore, you are answering it with a “they must have known!”

I just don’t see how you can know this. (For context, I work at JPL and interact regularly with some of the Perseverance team, but nobody from the helicopter team, which was very small.)

Some context. The helicopter was originally quite controversial because it had no clear science rationale (mission science requirements did not imply a need for a helicopter). My understanding is that it was put on as a tech demo and (paraphrasing) cool thing. The lab director at the time was a proponent.

Of course it was a huge success.

Because of its precarious status, it was not designed to fly a lot. Planning flights takes a lot of resources, coordination with the rover takes resources, etc. Bandwidth, time, and people. My point is that the “plan” was kept small partly because these ancillary costs had to be kept down. This would hold even if the hardware could last forever. The tech demo must not get in the way of the prime science mission, which had already been planned.

The use of COTS components is another factor. The effects of radiation on microprocessors (in the Mars-surface or deep space environment) are not really well known, as a practical matter. There is just not much engineering experience with how bad it can get. So the longevity of the Snapdragon was (AFAIK) highly uncertain, not to mention whatever other COTS components were on it.

Hope this context is helpful.