I believe the image he posted were Silver frags, not Platinum, and I am in the same boat as him… I have tons of gold fragments/platinum fragments, but have run out of bronze/silver fragments. At least Bronze frags are farmable but silver frags are exclusive to solo raids…
You said to switch to and upgrade a different class of hero, and normally I would and this would not be an issue, but this month’s faction heroes are ALL energy. The Devs said it themselves that upgrading these heroes would be most beneficial to our success in events for this month and I would like to upgrade these heroes exclusively to prepare for later events, but requiring 10 silver frags per silver bar and 30 per gold bar for 5 different heroes is a lot that is required for this item’s current means of availability.
With this being said, I think a more appropriate reward for this Solo Gauntlet Patriots Blitz event would have been to reward Energy canisters/fragments since those have become a huge bottleneck item when it comes to upgrading, and especially since it is an event that highlights them specifically. Just a suggestion for future events 
Cheers!