Shameless plug!
https://boards.na.leagueoflegends.com/en/c/gameplay-balance/jFUYaI1s-riots-new-crit-thoughts-are-behind-the-times-and-regressive-can-we-please-stay-modern
Marksmen itemization is not bad, it simply feels awful due to marksmen having a 4 item core to be properly effective. Riot wants them to be a lategame class, but they also should not have the best early game out of everyone else because of it.
Their attempt to rectify this was to slow down power spikes and shift around some power (the current IE, when you do the math, does similar damage to the old IE. Its basically -20 damage per 100AD after 200AD). It was a good start, but with riots official endorsement of storm razor as a marksman item marksmen mains have once again fallen into the cookie cutter looking for their 'old oneshot at 2 items'.
The way you fix the current marksman itemization issue is to reduce their itemization to three items from four. You can do this by simply reworking storm razor, as ive went over in my above linked thread.
Marksmen mains also want to do tons of damage with IE, so you can rework IE so that over-crit (crit above 100%) becomes crit damage, letting them forgo flexible item slots for mOaR dAmAgE instead of true damage.