It really depends on how much extra you will need to spend in order to get Silver....will it be worth it? One return trip in WT+ will bag you enough miles to upgrade the overnight SFO-LHR flight the next time you fly so that'll get you a flat bed, lounge access in SFO and the Arrivals lounge when you arrive at LHR anyway.

Work out how much it'll cost you to get Silver. If it's a realistic amount then go for it and see what benefits you receive. Then you can make your own mind up as to whether it's worth retaining.