Δαφνη Ευτυχια Αρθουρος

Apparently nobody much uses the complex binyanim in Modern Hebrew, as is the case, I'm told, with things like the masculine numbers and that sort of thing...

I'm curious about your source. I went to bilingual (English/Hebrew, including native-language teachers and some classmates) schools from pre-school though High School and was at various ages able to fool people into thinking I was a native Israeli, and none of this matches my experience.

"the complex binyanim"? Which are those? There are are roots which have standard meanings in only some of the 7 binyanim, or mean different things in different binyanim, so if you don't use certain binyanim you will lose parts of your basic vocabulary as well. To illustrate with two very common, very basic verbs: ktb 'write' is usually used in the qal binyan while dbr 'speak' is usually used in the pi'el binyan. If someone were to stop using either of these binyanim, they would lose some very common, very basic vocabulary. Some binyanim are used more than others, but all have some basic everyday words.

As for not using masculine numbers, they don't get used much for counting (as in counting on fingers) but if you use a feminine number with a masculine noun it really does sound wrong/non-native.