I have seen "scarcely" in older fiction. Not a common word in my social circle. "Rarely" is common as an adverb of frequency. -- He rarely reads. "Barely" is common as an adverb of quantity. -- He barely discussed his divorce.

You can use the phrase ‘scarely ever’ in place of rarely, as in ‘he scarcely ever gets home before 10’. Without the ‘ever’ though the meaning changes, in your example the equivalent word for scarcely would be ‘hardly’, as in ‘....are scarcely/hardly foreseeable’, i.e they can’t be forseen, it is not credible to assert that they are foreseeable.

You can use scarcely ever to talk about things that rarely happens :)

"Scarcely foreseeable" in your example sentence suggests to me a meaning of "difficult to anticipate"...that it was difficult to see signs in advance that this outcome would happen. If your intended meaning is to say that considerable changes in the stock market are expected to happen rarely, I think it would be better to write the sentence differently.