I don’t know what other protestants think but I prefer this definition:
Sola scriptura teaches that the Scriptures are the sole infallible rule of faith for the Church. The doctrine does not say that there are not other, fallible, rules of faith, or even traditions, that we can refer to and even embrace. It does say, however, that the only infallible rule of faith is Scripture. This means that all other rules, whether we call them traditions, confessions of faith, creeds, or anything else, are by nature inferior to and subject to correction by, the Scriptures. The Bible is an ultimate authority, allowing no equal, nor superior, in tradition or church. It is so because it is theopneustos, God-breathed, and hence embodies the very speaking of God, and must, of necessity therefore be of the highest authority.
If we are going to determine what apostasy is and when it occurs we need a criteria to determine this. Fallible men in a fallible church is not an adequate criteria or rule. Only the inspired-inerrant Word of God can help to determine what apostacy is and when it occurs.