Since many attractions have a reduced rate for Seniors, I wonder if it is still advantageous to purchase a Lisboa card?
It depends on what you want to visit. You likely will need to determine which museums/attractions you want to visit, which ones have the 50% discount and do the math. The Via Viagem card for public transport quite likely will be the best option..
Also check on free museum days. The Gulbenkian is fantastic and free on Sundays.