The fare basis is immaterial for anything other than to, first, establish your fare, and second, to state the rules the pax is bound by. Lounge access is based on the COS of the flight and not the fare basis of the ticket, just like miles are based on the COS of the flight and not the fare basis of the ticket. If I have a first class fare basis, and fly coach on a one-class plane, I will get coach miles irrespective of the fact that my fare basis is F or A or whatever. Same with lounge access. If I fly to Hong Kong in first, stay a few days, then fly to Singapore in business, then I will only get access to the business class lounge even though the fare basis of my entire ticket is F. If I fly coach, I people watch with the masses.

Just to add, this is why pax that upgrade will get lounge access. The fare basis on an upgraded ticket is still the original fare basis. If you buy a Y ticket and upgrade to business class, your fare basis remains as Y. Your *booking code* is what changes to U or whatever the individual airline uses. That's the code that is used to determine perks like baggage, lounge access, fast pass, etc.