Yes, there are some churches of the elder remnant still hobbling along, though you are right, Mike, they have long ago compromised their stands for the Lord, especially over the Sabbath. Few can withstand the pressures of years of papal or Protestant oppression to conform, wither or die.

The Waldensians still exist (as a form of Presbyterian/Methodists) in Valdese, N.C. and in Rome and S.America.
The Assyrian/Chaldean church has mixed groups here and there, mostly of an Orthodox blend, and the famous St. Thomas christians are still thriving in Kerala, India. The Moravian church is still alive, as well.

I have visted all of these churches in person, or have met and talked with church memebers. I know that they have a rich history which few of them even know about.

But I see your statement below as a bit unclear:
"Jesus will return when all the world has either joined or refused to join the SDA Church."
Do you mean that there will remain a well-organized, structural church like today's at Christ return, or do you actually mean to say "...or refused to accept the truth of the Third Angel's message, upheld by the persecuted and scattered companies of the commandment-keepers"?

The gathering of God's faithful believers into the Remnant Church began in 1844. Had these early believers worked earnestly the world would have been warned and Christ would have come long ago. Everyone everywhere would have had ample time and opportunity to decide for or against joining the ranks of God's remnant church.

Ev 695,696
"Had Adventists, after the great disappointment in 1844, held fast their faith and followed on unitedly in the opening providence of God, receiving the message of the third angel and in the power of the Holy Spirit proclaiming it to the world, they would have seen the salvation of God, the Lord would have wrought mightily with their efforts, the work would have been completed, and Christ would have come ere this to receive His people to their reward. But in the period of doubt and uncertainty that followed the disappointment, many of the advent believers yielded their faith. . . . Thus the work was hindered, and the world was left in darkness. Had the whole Adventist body united upon the commandments of God and the faith of Jesus, how widely different would have been our history!

"It was not the will of God that the coming of Christ should be thus delayed. God did not design that His people, Israel, should wander forty years in the wilderness. He promised to lead them directly to the land of Canaan, and establish them there a holy, healthy, happy people. But those to whom it was first preached, went not in "because of unbelief." Their hearts were filled with murmuring, rebellion, and hatred, and He could not fulfill His covenant with them.

"For forty years did unbelief, murmuring, and rebellion shut out ancient Israel from the land of Canaan. The same sins have delayed the entrance of modern Israel into the heavenly Canaan. In neither case were the promises of God at fault. It is the unbelief, the worldliness, unconsecration, and strife among the Lord's professed people that have kept us in this world of sin and sorrow so many years.--Manuscript 4, 1883.

LDE 211.6
"In heathen Africa, in the Catholic lands of Europe and of South America, in China, in India, in the islands of the sea, and in all the dark corners of the earth, God has in reserve a firmament of chosen ones that will yet shine forth amidst the darkness, revealing clearly to an apostate world the transforming power of obedience to His law. Even now they are appearing in every nation, among every tongue and people; and in the hour of deepest apostasy, when Satan's supreme effort is made to cause "all, both small and great, rich and poor, free and bond," to receive, under penalty of death, the sign of allegiance to a false rest day, these faithful ones, "blameless and harmless, the sons of God, without rebuke," will "shine as lights in the world."--PK 188, 189 (c. 1914).

As Jesus is returning and the unsaved are lamenting their lost condition they refer to those who preached the 3AM's as Seventh-day Adventists. Although SDA's will be unable, in most places, to meet in SDA owned and/or operated buildings during the little time of trouble (just before probation closes) all those who refuse to comply with the MOB laws will be known and persecuted as Seventh-day Adventists or accused of being a Seveth-day Adventist.

Mar 290.3
"The people who have braved out their rebellion will fulfill the description given in Revelation 6:15-17. In these very caves and dens they find the very statement of truth in the letters and in the publications as witness against them. The shepherds who lead the sheep in false paths will hear the charge made against them, "It was you who made light of truth. It was you who told us that God's law was abrogated, that it was a yoke of bondage. It was you who voiced the false doctrines when I was convicted that these Seventh-day Adventists had the truth. The blood of our souls is upon your priestly garments. . . . Now will you pay the ransom for my soul? . . . What shall we do who listened to your garbling of the Scriptures and your turning into a lie the truth which if obeyed would have saved us?"