Yes, that’ll work. Some kind of guarantee that, notwithstanding heresies, the Church itself would be preserved in orthodoxy until the end of time.
Well, the problem for me is that IMO Matthew 16:18 does not provide that kind of promise. But knowing that you're coming from that end, we can still have the discussion around the concept, so long as we're arguing the concept itself, and not arguing the exact interpretation of that one verse.
I would define "the Church" as being "all of those who obey the Great Commission, and who make the same profession as Peter." To a lesser extent, I might argue that what constitutes "the profession of Peter" includes what's contained in the orthodox creeds. I know that you and I would not agree on the list of what constitutes the "orthodox creeds" - I do think that we could agree on a subset of that list, however. Moreso (for the sake of the evangelical lurkers reading this) I am emphatically not suggesting that a profession of the creeds is required for eternal salvation. I am suggesting (and note that I only said suggesting) that a profession of a creed is required for covenantal self-identification with the earthly, visible Church.
The important point to get out of this is that I'm defining the church as "those who believe the profession", not "those whose ancestors believed the profession." The absolute common element I want the reader to get here is that, from age to age, I believe in the preservation of a creedal continuity, not an apostolic continuity. I believe that there can be a discontinuity in apostolic continuity, while still preserving a church "in orthodoxy" via creedal continuity.
Think of it this way: I believe that there will always be a Church that makes the same profession that Peter does in Matthew 16:16. I believe that God will "raise sons of Abraham out of the stones" if need be to do this, and that such sons are legitimate - every bit as legitimate as those sons who can trace an unbroken temporal lineage back to the first century church, provided that each makes the same profession as Peter. And I'll go one step further than that - I do not believe that scripture demands that these "sons" need all belong to a single organizational structure. Adherence to the profession defines whether they are truly members or not, whether said professors recognize each other or not. Creedal unity does not necc. produce ecclesiastical unity. IMO it can, should, and will if the professors are consistent in all areas of their beliefs, but I would not negate their legitimacy if they are not in ecclesiastical unison. People are imperfect.
That said (whew), we still haven't fully defined what "preserved in orthodoxy" means, or what constitutes "the profession of Peter", i.e. or what bare minimum measure of orthodoxy (profession) is needed to qualify as being "preserved". I'll leave that to be addressed in another post.