The reason I bring in that comparison (and you can include Jota at 8m as well) is because already within 1 GW that was the question that a lot of people with big at the back were asking, that should they drop a premium def especially Robertson to free up a Liverpool spot to get Darwin because that's the most direct route anyone with big at the back will have to get a third striker in that bracket. Yeah it will need a little adjustment elsewhere but that's a rather direct move. Even more close in the case of Jota, and actually cheaper in case of Toney. This question will likely be asked again once they are back, or once multiple 8m mids start returning. It's essentially the choice that you need to make.
I absolutely don't disagree with going back at the back being a great strategy to start this season. The options were clearly reliable as well as underpriced for the most part as you say, but I'd say the idea for most was to wait and see which attackers emerge and slowly start dropping one or two of those big defenders and moving to the attackers which is what is happening now (and that's when Darwin got sent off, who would have been the biggest breaker of big at the back). So it will still be keeping the likes of Trent/Cancelo/James (probably 2 max) and loading the rest with attackers which is more at par with how FPL generally goes.
For the 5m defenders, hell yes they are underpriced and that's the case every year but that's not big at the back. We are talking about having at least 2 7m+ and 4 6m+ defenders which is what a lot were confident on being a season template, but it is possible it can turn back to the usual 343s and the like. The likes of Trippier, Saliba/Gabriel, Doherty/Perisic etc are still gonna be great value regardless and aren't exactly premium defenders.
Only narrative that I felt disagreeing with really was that there was no value in 7m+ attackers (strikers or mids) as compared to 6m+ defenders. And that was largely down to the let down of last season. Because otherwise pretty much every season has 7m+ strikers at least 2 or 3 of them firing at all points in a season and people just get them. All your DCLs, Ings and what not, that's what's happening with your Toneys, Mitros etc now. heck people wildcarding now are interested in Welbeck.
You don't need much from strikers to get the bandwagons going, thats where the biggest loot in FPL always lies. Even last season is a clear indication with the whole Dennis/Broja bandwagons. Chelsea probably get Auba as well which will add another one.