"flat data plan for $5/month at LTE speed with this type of smart phone"
Flat data plans are gone forever I'm afraid. We might get more data for less money over time but there is no way they are going back to unmetered data.
You're obviously in the US I take it? I'm in ireland and have a UK sim card too. Both my providers (granted, same provider in both countries) give me unlimited data, for 20 euro/25 pounds a month. I also get calls and texts included in that.
Not sure how it is in the USA, but unmetered data plans do exist - here in Ireland it's the equivalent of ~$27 for unmetered 4G data (with a certain limited number of calls and texts included)
Un-metered data in Germany as well. Only the speed gets throttled when I reached my quota. This is not much of a problem with so many open wifi access points around these days.
Flat data plans are gone forever I'm afraid. We might get more data for less money over time but there is no way they are going back to unmetered data.