I think it's interesting that you called out LH F in your post. A lot of the advice I would offer depends a lot on whether you are looking for a 3-cabin First level of service in your travel, or if Business would do.
If LH F is the level of quality you want, you should not bother with loyalty to United. UA F is a dying, inferior product with a nice seat and absolutely nothing else (besides the fabled soup) to distinguish it from J. Polaris is nice, but doesn't hold a candle to a true F product. Fly LH F, SQ F, CX F, etc. and don't worry about loyalty.
However, those products can be extremely expensive even when compared to J class products. In the simply J world (and I will get a lot of flack for saying this), UA isn't all that bad. I agree with the decline in GS benefits you cited (although you still have
hugely significant award travel benefits - in summary your saver Economy inventory is T and saver Business is R, and you can force award space for your spouse). But I would find it hard to say things are that much better anywhere else. Unless all your international travel is to a select city set, you're not likely to make or benefit from top status on a non-US carrier. For all its decline, UA GS is still an extremely valuable status, if only for the ton of upgrade instruments and award travel benefits that can save you money especially on leisure travel. If you're finding that, without really forcing it, your travel qualifies you for GS, I still think it's the best bet out there.
Especially if you like taking family vacations, the upgrade and award travel benefits of GS are hard to match on any airline, anywhere.