You missed the heart of the issue. Unlike saving belief, shallow, temporary belief that has no root, produces no fruit and withers away is not rooted in a regenerate heart. How can no depth of earth, no root, no moisture, no fruit, represent saving belief? It doesn't so there is no loss of salvation here. Also the same Greek word for believe "pisteuo" is used in James 2:19, in which we read that the demons believe "mental assent" that "there is one God," but they do not believe in the Lord Jesus Christ (Acts 16:31) and are not saved.
John has portrayed people who "believe" (at least to some extent) but are clearly not saved. There is a stage in the progress of belief in Jesus that "falls short of firmly rooted and established belief resulting in salvation." As we see in John 2:23-25, in which their belief was superficial in nature and Jesus would not entrust/commit Himself to them.
Also, in John 8:31-59, where the Jews who were said to have "believed in him" turn out to be slaves to sin, indifferent to the words of Jesus’, children of the devil, liars, accused Jesus of having a demon and were guilty of setting out to stone and kill the one they have professed to believe in. We can see at best, these Jews believed in Him (based on their own misconceptions and expectations) of Jesus, yet upon gaining further knowledge about Jesus through His words, we see they did not believe unto salvation and become children of God (John 1:12; 3:18) but were instead children of the devil.
In the case of the thorny ground hearer, the plant was never firmly rooted and established in which it could produce fruit (even though there was motion and movement toward becoming an established plant) but it was choked out before reaching its desired goal because the cares of this world, the deceitfulness of riches, and the desires for other things entering in choke the word, and it becomes unfruitful/bears no fruit, produces nothing. (Mark 4:19). The word is choked and therefore does not produce a renewed spiritual life in the person.
IN CONTRAST TO - Mark 4:8 - But other seed fell on good ground and yielded a crop that sprang up, increased and produced: some thirtyfold, some sixty, and some a hundred. Luke 8:15 says, But the ones that fell on the good ground are those who, having heard the word with a noble and good heart, keep it and bear fruit with patience. *Only the 4th soil produced crops of any size and was referred to as "good ground" and produced a crop (fruit) and there is no mention of choking or withering away afterwards.