It was a little disconcerting when Lucas said "chances are that [Indiana Jones 5] will be not well accepted much more than it will be well accepted, so there's not a lot of impetus to do it." Most franchises have a low threshold for acceptance: if things go sour, given time, fans will accept a new cast (Star Trek), new character (Bourne), new director (Pirates), or the exact same story over again (Spider-Man). Some franchises are allowed mulligans (Bond) or the chance to stumble towards the final conflict fans were promised (Star Wars).
Try these sortable tables. Find a franchise with a smaller relative drop in quality than Indy, if you can. Toy Story is perhaps the only better open-ended large franchise, while Lord of the Rings and Harry Potter followed their established arcs smoothly. Final Destination bucked the trend and closed (?) on a critical high note, unheard of for a horror franchise, but it's nowhere near the same league.
Most who complain and protest are probably just being protective of Indy and their youthful free associations. Some of us like how the franchise, like the character, is unfairly pitted against seemingly insurmountable odds.