Well, Potato, we can look at this from the Protestant viewpoint. So: Their statement above has to be true, for if it is not true then Protestantism is wrong from the first moment, and since that can’t be right, then the death of the church must be assumed.
Also Mormonism, the Jehovah’s Witnesses, and every Christian religion or religion claiming to be Christian: they all have to be two contrary things at the same time. They must be Christ’s true church which failed but somehow survived. That survival turns out to be in some non-actual or non-historical way, such as potentially, or virtually, or in the heart of God, or spiritually, or underground, etc. That break is the great Protestant Assumption, a doctrine which is simply assumed, for otherwise all the protesting is vacated before it even gets started.
For if Christ’s Church never failed, as He promised it never would; if He has always been with her, as He said He would be: then no matter what good points Protestantism has, it is vacant of validity at its roots.
It can be useful to concede what is obvious and true about their thinking: It certainly would be important if the true Church vanished after St. John the Apostle. Yes, indeed, for sure. For if such a Great Apostasy (mormon terms) occurred, then the resulting great vacancy needed to be filled by the truth once again. And that would make the news of (Protestantism, Mormonism, JWism, etc., whichever) very good indeed.
After agreeing to this in principle, you can then move on to saying that, however, if there was no such break; if in fact there is continuity, then of course, though there may have been plenty of room for reform and renewal, there was/is no room for Protestantism et al.
I tend to ask things which will focus on this great apostasy, this terrible failure of Christ’s Church to continue: under which pope (what name) did this apostasy occur? What were the particular issues? What were the church councils saying before and after the apostasy? Were there battles? Was there land won and lost? What land? What were the sides in the struggle? Led by whom, that is, what were the names of these people? What cities hosted these councils/battles? How about some dates?
In other words, where is your positive evidence that such a break in continuity even happened?
Otherwise it’s wishful thinking, and one huge cover-your-backside assumption in order to backward-engineer the justification for not being in Christ’s ever-continuous Church.
This is a conversational approach one can take. Of course as a conversation, it doesn’t tend to go on very long. Anyway, hope this and the other answers on the thread are helping.