Absolutely. But few reliable records were kept regarding voluntary attrition and it's hard to know what was in people's heads and hearts as they started out, and what happened to their thinking along the way. Remember that all the pioneers started out because they had "a story" in their heads of what to expect at the end of the trail. Over a period of 6 months of grueling travel, they frequently heard a better story from other fellow travelers, and it was not uncommon to jump to another sub-group to go to a different destination. Remember the Oregon trail was actually a network of trails, with many tempting branches to take along the way. Also many other trails went by other names that used at least a portion of the same route. A pioneer might leave Illinois or Ohio headed for Oregon, then decide to change destinations based on stories heard from other fellow travelers on the Mormon trail, the Overland trail, the Army fort resupply trail, and the California trail - all of which partially followed the same route. When this happened, they tended not to write it down, they just took a different turn in the trail.
Here's what we do know. We know up to 400,000 people took the trail. Pioneer diaries and records also kept pretty good track of deaths and from that we can determine death attrition. We know about 7% didn't make it due to disease, accidents and Indian attacks (in the lexicon of the day), so that is 28,000 people that didn't make it and were buried along the trail. Most pioneers who got sick did so in Nebraska, where reused campsites with prior human feces in the drinking water caused huge outbreaks of cholera. If you made it out of Nebraska and across the border into Wyoming and were still standing on two feet by the time you got to Fort Laramie, the worst was over disease-wise, and you had a good chance of statistically making it to the end of the trip. Some of these deaths however stopped all forward movement of the survivors. For example if a wife lost her husband and/or children along the trail she would typically be without a means of support and unable to claim, clear and manage a homestead of her own if she made it to Oregon anyway. After burying her husband, she would most commonly seek immediate shelter and employment somewhere near the closest town or Army outpost doing laundry or other domestic services. Others saw entrepreneurial opportunities along the way, and decided to just stay right there and mine the pioneers yet to come. As an example, traveling the trail involved dozens of river crossings, which in the early years of the trail all involved getting wet, and many drowned. Many travelers saw an opportunity to build a bridge or ferry service at a particular spot and so stayed put and did so. Other sharp-eyed entrepreneurs noticed all the horseshoes were wearing out at about the same spot on the trail and that a blacksmith was needed at that location, and so on and on it would go for all the trail infrastructure services. Eventually as support services increased, travel along the trail slowly got easier and faster, and so there was less attrition as time went on. We know some of the early wagon trains in the 1840's only arrived with 50% of the people that started out, and that some of the later wagon trains by the 1870's were arriving with 90% of the travelers intact . Putting this all together, it would then seem like an average wagon train would probably lose 7% due to deaths, and another 20 to 25% or so for other voluntary reasons and diversions. Putting it all together, it would suggest that about a third of those who started out would have melted away from one reason or another by the end of the trip.