Breaking this into three distinct cases:
1. To the present, it has never occurred to anyone they could get scans of the original Japanese publications of manga, translate them, and shop those translations into the images (that is, the idea of scanlation was never conceived):
There would be a lot of pirated scans of what official translations do exist, and then one day some brilliant person who is fluent in both Photoshop and Japanese and who is privately reading some non-licensed series will think 'wait a minute, I can do this myself and make it available to everyone.' And then we're off to the races, and it's only a matter of time before we're shitposting in the general forums on the alternate universe MangaDex.
2. Some heavy-handed federal law is passed which makes scanlation punishable by death, in an attempt to put a stop to it:
Yeah, it's the internet. People will still be doing it for series they really like, though they will be careful about not getting caught. (Compare music sharing after the RIAA started suing people's shorts off.)
3. Someone waives a magic wand, and from that point on anyone who tries to scanlate has Spontaneous Thermal Paste Combustion Syndrome afflict their computer:
People quickly figure out how to do the work on paper, leading to UNDERGROUND MANGA 'ZINES. So many 'zines. Every aging rocker/punk/riot grrl is pressed into divulging their suddenly priceless knowledge of preparing these publications, and the market for old Xerox machines explodes. The overnight increase in mail brought on by vast quantities of fan-translated manga on paper single-handedly pushes the USPS back into profitability.