I would be willing, were it possible to reliably test the hypothesis and I hadn't already spent it as a down payment on a car, to bet the entire $7,000 insurance check my wife and I just got, that less than 20% of people who played healers in older editions enjoyed fights where their only contribution of any meaning was healing. Where every action was spent healing. I am absolutely certain that most people don't think that hitting the "heal bob the fighter back up to full as my entire turn" is especially fun, compared to "get bob the fighter back into the fight and also use turn undead to scare away some of the zombies or wade in and smack down the demon that raised them because I'm a war cleric or a paladin or whatever.
The biggest problem with healing word is simply that it precludes casting another leveled spell with your action, and clerics have boring cantrips.
Hell, most people I know consider a devotion paladin more fun as a healer because they can use healing word and lay on hands in the same turn, while clerics can't do anything especially powerful in the same turn they use healing word.
Of course, healers get even more fun at high levels when you get group heals and healing auras that also buff, and stuff like that, but it's nowhere close to how fun healers were in 4e, where you could hit an enemy with holy fire and while the fire lingered on them your allies got healed every time they hit the thing and fun stuff like that, and your minor action heal didn't interfere with casting powerful prayers.