What's threatening the BSA right now is not so much a legal challenge (which they've always been able to win in court), but a societial challenge being fought at the fund-raising and membership recruiting levels. Besides, they'd have a hard time doing this because 1) they accept Scouts and Scouters of all religious denominations, as well as those who have no membership in any religious denomination as long as they personally profess belief in a higher power, and 2) a large number of units (some 45%, I believe) are sponsored by secular organizations.