Drew,
You're both correct and incorrect.
The simplification in 1955 did elevate Advent Sundays to Doubles of First Class. So, yes, the Immaculate Conception was replaced with the Sunday in 1957. Here you are correct.
The 1960 Rubrics established a new ranking system and table of precedence to remedy this problem. As shown above the system ranks the Immaculate Conception higher than the Sunday, so if following the 1960 rubrics one celebrates the feast, not the Sunday. Here you are incorrect.
Until 1956 the feast takes precedence. From 1957-1959 (in practice only in 1957) the Sunday took precedence. Since 1960, the feast again takes precedence.
No one follows the 1955 changes without the 1960. If they dislike the 1960, they don't revert to 1957. If they like or tolerate the 1960, there is no basis for reverting to an archaic system. Thus, there are no people today who would hold this "middle position". And if none of these in practice exists, then neither would the Immaculate Conception be omitted or transferred.