It is difficult to change the vernacular, and as many said here, language is dynamic and ever-changing. Therefore, it is perfectly acceptable to say shabbosim and taleisim and shtusim.
That said, I suspect that even the “who cares” contingent here will correct the ba’al kore if he leins שבע שבתים תמימות תהיינה. So, sometimes it matters, and when it does, it matters alot.
One of the most amusing mistakes found in the mefarshim is on the first page of the standard Ritv”a Al Hashas found in every standard beis medrash and Jewish home of the learned. It says that the new edition has corrected all the טעותים from the older editions. I always laugh at that faux pas.