Remember that not all metamask user is using Uniswap or other DEX that needs extreme transaction speed in case of network congestion. The reason why they might not save the gwei is because they are making sure that people will not mistake on using higher fees. If a user want to transfer fund during congestion and want the transaction faster than the default fasted. They will manually adjust the gwei but what if they will send again in normal speed but forgot to change back again the gwei in default? It will cost 20$ ~ 50$ for a normal transaction if the feature that you want implemented.
In addition, it should be implemented on the side of the dapps, in this case, to the Uniswap themselves, it is true that it could incur your fees on normal transactions outside of the uni swap when this feature was added. And what disaster it could get further is that transaction fees on Ethereum will be high as people always choose the option to send the fastest transactions on the main net, as a result, it will be like bidding, the higher the fee, the faster it would be transferred. They have already set the standards for the most efficient way of managing the gas, I think that is enough and it is on our end to adjust.