The Alaska Visa is a good card for folks who fly Alaska, the $50 companion ticket is really useful, but your travel plans are a bit far off and somewhat unclear.

I only carry my traditional Amex because as a charge card rather than a credit card Amex will let me use it on six-figure work expenses ... better than foregoing all those miles.

Frankly I wouldn't keep the Amex Platinum but for that.

Membership Rewards isn't as good as Diners Club Club Rewards (since Amex has fewer partners).

Starwood is better still (no points transfer fee to airlines, built-in bonus for transfers of 20k points to airlines, plus those Starwood points are pretty useful in their own right).

That leaves reasons for keeping the Amex as lounge access (which you say you don't use, and could be acquired more cheaply anyway), Best Value Guarantee (which IIRC also comes with the Starwood Amex), and the FHR program (good if you stay at Ritz Carlton or Four Seasons type properties, I value the FHR program, but I'm guessing you aren't using it much now). Oh, and the Concierge and the Amex Plat concierge is better than the VIPdesk folks but Diners Club and many Visa Signature products do come with concierge service.

Personally I'd dump the Amex Plat and its $395 fee and use the Starwood Amex as my primary card... and then ask myself whether I spend much that can't go on the Amex. Still a good idea to have a Visa/MC but don't pay a fee unless there's > $15k/yr that you can't put on the Amex.

Put another way, since you're banking miles rather than using the non-mileage benefits of various credit cards currently, why are you using a card other than the Starwood Amex?