According to my uncle, who is a Conservative rabbi, the psak was issued for a few reasons. It should be noted that he does not like it, although were he to attempt to pasken that his schul not hold like it, he'd probably be fired.
In the decades immediately after the War, many of the Jewish communities in inner-city neighbourhoods started moving out to the suburbs, along with their schuls. When a particular schul in Detroit moved, it left behind its older membership, who were now several miles away by foot. The rabbi (whose name I do not recall) paskened that his congregants could drive to schul on Shabbat, due to the new distance and the advanced age of many of those who remained in the older neighbourhoods.
Thus, it was meant to keep older and infirm members in, rather than to bring more affluent members back.
My uncle's analysis is that this let the genie out of the bottle (so to speak). While the intention is clearly to make ritual observance easier for the non-religious masses, the result was a general tendency towards extraordinarily lax standards for the laity and a reinforcement of the divisions between them and their JTS-educated rabbanim. JTS rabbinical students of that time were held to nearly Orthodox standards, up until the acceptance of women into the rabbanut.*
*This last point is an observation of my father's. He attended the List program at that time, although he was at the time (and still is) strictly Orthodox.