WSS is problematic, very hard to produce a first class production. Casting requirements: Youth, star quality (for Tony, Maria and Anita), major singing ability, and even more dancing ability. It does not belong in an opera house which is going to sacrifice dancing and likely star quality. There are musicals that can go there but I'm definitely not convinced that WSS is one of them.
Also duly noted about Gilbert and Sullivan. The D'Oyly Carte Opera company had its heyday in the 50s and 60s when they fielded opera quality casts. The round of London recordings from the company, first stereo ones show the company and Gilbert and Sullivan at their best.