The 1950s impacted very heavily on KOTCS, so if Lucas continues in that vein, then he'll probably be looking for more contemporary references for the next one.
Part of the difficulty in initially accepting KOTCS was not so much Indy's age, but the period itself. I have the notion that the 1930s were an age of wonder nd mystery, which the pulps accentuated as an escape from the very real impact of the economic depression.
The 1939-1945 war years accelerated modernization, and the world became smaller through technological advances. Some of the mystery was eroded. I always think of 1945, at the end of the war, as the beginning of another modern age. Therefore, it was hard to see Indy in this new world. Harder still to see him in the 1960s, though this is where I expect he'll be in Indy V.
It's also a transition from the stylish art deco world of the 1930s into the age of the concrete high-rise block. A time when the world was losing its charm (seen personally as a mythical golden age), and succumbing to brute ugliness. Indy not only has to battle his own age, but the age he's forced to exist in.