In another question I ask about the ideal flour, sugar, butter ratio to use in crumble. It appears that the amount of sugar people add in their recipes depends more closely on how much butter is added than how much flour is.
I theorised that butter reduces perceived sweetness, and I remember reading that fat acts as a taste inhibitor. I googled a bit to check this, but some say it can enhance taste.
So, does butter reduce perceived sweetness or enhance it? Surely it can't do both.
You've got to understand what the purpose of the ingredients is. In a crumble or streusel topping (my go-to in the category is an apple crisp), the (gluten/wheat) flour, butter, and sugar are actually all binders/structural components. Taste/flavor is part of it, but as in your experiments in the other answer, you really can't vary one element wildly (separate from the others) without significantly changing the results.
With that in mind, the butter in a crumble/streusel topping will affect the tenderness of the crumb (similar to cake/pastry). Tenderness will also be affected by whether you used cold, melted, or softened butter (or a non-butter shortening, like margarine or lard/Crisco). The tenderness goes to mouth-feel, which will affect how/when your tongue picks up on the diff flavors.
Similarly, the temp/state of the butter/shortening affects how & how much the sugar will dissolve/interact with it. Depending on what type of sugar you're using (granulated/white, dark/light brown, raw/demerara, liquid sweeteners like honey, corn syrup, molasses, or agave), cold/melted/soft butter will just have a different holding capacity for sugar, which cooks can take advantage of to "super-sweeten" or "under-sweeten" given fundamentally the same amount of sugar.
And of course in a crumble, your perception of the sweetness of the finished topping will be impacted by the filling, since there's (delicious!) flavor interaction during cooking.
So it comes down to ingredient preparation & choice and cooking techniques - not just raw proportions. Cooking is a sort of applied chemistry - sometimes how you put things together is as/more important than what you put together.
