I could agree with that easily enough. After all, a basic tenet of orthodox/evangelical universalism is that the only unforgivable sin is the one that is not repented of.
In this case, a sin not unto death would be one that the other person is repentant of but still tends to habitually do, or he tends to fold under temptation, or whatever; but he does know it’s wrong and (this is the key point) is seeking to be free of it. (Several of my own persistent sins fall under this category.) A sin unto death would be one the other person is unrepentant of. Not only would such sins be somewhat feasibly identifiable by us as external second-party observers, but it might actually be a sin to ask God to give life (in the sense of the zoe eonian) to someone who persists in being unrepentant! (This should be distinguished from those who are sinning but don’t realize yet that they are sinning.)
Not that this would solve the translational difficulties. But the typical translation is far from being completely fallacious, either.
From the standpoint of universalistic soteriology (as developed elsewhere), what should be prayed for is that God would lead the sinner-unto-death to repentance. The final chapter of RevJohn is (one way or another) an excellent poetic-puzzle example of this process; and indeed shows that God expects us to help Him with this goal, encouraging those who still love their sins to repent and drink of the water flowing from under the throne of life, freely and without cost (major OT imageries there, as well as NT Jesus links), so that they may obtain permission to enter the never-closed gates of the city and eat of the tree of life, the leaves of which are for the healing of the nations. Washing of garments in the river is included in the imagery, too.
Arminians could easily go with that interpretation as well, so long as they read the final chapter of RevJohn as being a sort of flashback to what we’re supposed to be doing now before the final judgment after which it’s too late (and the never-closed gates are closed, or something. )
Calvs would have the hardest time with that interpretation, but would obviously have other interpretative options to choose from. (No hope at all for the one sinning unto death, etc. They’d either have to read RevJohn 22 as being a flashback picture of God persistently saving the sinning elect before the final judgment, which is why the gates are never closed for them; or they’d have to ignore/wildly-redefine the salvific elements of the portrait altogether if they understand it to happen after the lake-of-fire judgment. I’ve seen Calvinists go each way. RCCs with Calv or Arm soteriological leanings either one, might easily consider this a portrait of the RCC version of purgatory.)
At the end of the day, 1 John 5 isn’t a decisive chapter for any of the three basic soteriology groups. The sin-unto-death verses are tough to translate, and positions already developed elsewhere can be fitted into the various translation options (sometimes into more than one option per soteriology.)