Julie,
First off, I happen to be quite lucky that IM estradiol valerate at a dose that's 1/4 what would be 'normal' transition dose not only affords me feminine estrogen levels and pre op also fully suppressed testosterone so I took myself off of spironolactone right away.
Spiro does have health effects and potassium balance is a touchy thing to be influencing. However, for most patients, the alternative would be very high doses of estrogen (anecdotally IM estrogen is better at suppressing T but I don't have any peer reviewed resources to hang that on). Before the availability of various anti androgens, that was the only option and taken orally, the implications to stress on the liver are significant, alternately, femininization was slow when you couldn't hit target levels of E, T.
I think the even more important element is availability of bio-identical hormones. Estradiol valerate provides the same net chemistry as natal females experience, micronized progesterone is by all accounts equally good. 20 years ago you were looking at conjugated estrogens derived from horse urine and medroxyprogesterone , both of which had significant health implications for many women (cis and trans). All of this results from far more sophisticated biopharmaceutical manufacturing processes (I've worked in pharma manufacturing research and can speak to what a revolution has already happened, to say nothing of what we'll see in the coming years.
Since I haven't needed finasteride, duasteride, I haven't researched those.