Game-wise, unless I am sorely mistaken, Rogues have never been thematically/historically been the class with the 'fast movement speed' shtick. And I will understand why other DM's will interpret it differently. I just don't think Rogues have thematically needed anything else aside from backstabby-stabbity, skill monkeyness, and shenanigans.
The new rogue is a good representative of the thief/rogue of previous edition. The triple dash --- while built into the 3rd edition as full round run, was never exclusively a Rogue's ability.
But hey. I'm just one guy, who thinks about what an ability was intending it to do.
I am sensing a lot of love for the triple dash. I just don't see it - in game history, intent, and flavor.
I still stand behind the ruling - Bonus Action: Dash does not stack on Dash. Rogues never needed it. Flavor from previous edition? Didn't see it either. Intent -- this is where I am not clear. But all I am asking for is that IF a DM rules to go against the stacking - you give them their due, and understand why they did it. Implement it at your own game table, or find another DM - it's fine really. Just respect the guy that rules contrary to stacking.
If your DM allows it - I'm happy for you. If your DM does not - at least I can say I understand why he did it. That's all it really is.
DM'ng and RPG's very nature is subject to individual interpretations.