Making cross-tier trades impossible means changing the way quests and Events are done. If a player with only T1 goods can't "gain a small amount" of T2 or T3 goods they can't complete the quest or Event. Most of my cities have all three tiers, and I still need to do cross-tier trades for goods I can't buy from the wholesaler. If cross-tier trades were ended, that would be a big incentive for me to play less, or quit all-together.
I have to say I totally agree that a ban on cross-tier trades would put a serious handicap on new players. I think we need to encourage the new players as much as possible. If we don't have new people coming in eventually things will stagnate and decline. I see how few of my neighbors who start out stick with the game. I'll say maybe 20%, and that's being very generous. I've also started finding Chapter 6 and higher cities that are abandoned as well. Many of these were active just a few months ago.
My point is that we need to encourage the new players that do stay and putting these glass ceilings up is only going to discourage them. The game allows cross-tier trades so why the need for restrictions that the game itself isn't even imposing? I think that perhaps many of the higher chapter players have forgotten what the struggle was like starting out. New players have enough disadvantages without more being imposed against them by their own FS.
I don't see a huge problem with cross-tier trading. If you do the math it works out, 1 Tier-3 is worth 4 Tier-2 is worth 16 Tier-1. I'll grant you that it's not a perfect analogy but I view cross-tier trading no different than someone offering you 5 singles for a 5-er or 4 5-er's for a 20. Sometimes you need Marble and other times Gems and one won't work for the other but with a cross-tier trade you can convert them. There is no loss of value, only a change of form. Value is only lost when producing non-boosted goods or trading outside 2-star trades.
Where I do see a problem with cross-tier trades is when that is all a person offers. I have a neighbor who only makes cross-tier trades and they are mostly Tier 3 for Tier 1 trades. I'm not even sure why they trade like this. I assumed they only had Tier 3 manufactories and were trading for everything else but it turns out there is only one non-boosted good that they are not producing themselves. For all the non-boosted manufactories they have they are still having to trade for what they need but the trades are up for days because no one wants them. Even if you're willing to take them, which I am, you quickly deplete Tier 1 goods when trading for Tier 3 goods so it's very self limiting. In this situation I think it is very detrimental to their city growth. Those around are deprived of potential trades but not as much as they are holding themselves back waiting for trades that aren't coming in because people can't handle them or just won't take them.