As much as I prefer saving my FGs for stamina, I can't argue the points you've made (but I'll try!); if you don't want to pay any money at all into the game then without the FG gachas, you will hit a brick wall with just event 5*s...but not as soon as one might think.
A good number of event 5* have decent stats and abilities, and since they are pretty easy to get to 5/5 will definitely outperform most, if not all, non-event 5* who are sitting at 1/1. I'm not sure that a group of 5/5 fully ampied event girls could take on later whaleship missions, but they will definitely get you through all other current content.
But, until you build up a good stable of event girls (don't forget the reissues!), I would probably still argue that FGs are better for stam to allow you to run reissue maps alongside current events. It definitely isn't exciting, but I think would get you the most bang for your FG. Right now, there are 20 permanent-reissued girls to get, one reissued girl and of course, the current event girl. That's more than enough to get you a team of full 5* units with enough variety of abilities and attack types (at least 5 of each attack type).