Actually, I would agree that losing the user for a benefit as huge as a +2 boost to offenses and speed would be meaningless. After all, people used to run suicide leads in 4th Gen (and occasionally now) simply to get Stealth Rock on the field. Suicide leads often just get up their hazards and then die, so their death is essentially meaningless as long as they get the job done. That would effectively be the case here, only there's no telling what else you could do with whatever Pokemon that gets that move. You might kill something, status something, do whatever damage to the opponent's team before possibly using that move late game and opening up for an easy sweep. Not to mention you aren't forced to run White Herb to prevent defense drops from being passed like with SmashPass, and you also only use up one turn instead of two.
In terms of competitive battling and competitive battling only, yeah, a Dark Link user could be a suicide lead that lets a mixed sweeper switch in and wipe the floor with the foe's team (or do something similar with its stat boosts).
Pokémon whose primary purpose is to use Healing Wish typically don't care what the foe does to them, similar to suicide leads that use entry hazard moves, and with the penalty that Dark Link has now, there's nothing that can be done to prevent people from taking advantage of the penalty.
On another note, dropping the +2 Speed boost entirely would make using Dark Link seem less one-sided than it would otherwise be. With +2 Attack, Sp. Atk,
and Speed, the recipient of Dark Link would pretty much be able to outspeed everything the opponent has and sweep them, yeah. But if the Speed boost were gotten rid of, even though the recipient does get a net +4 stat boost, the foe still at least has a chance to answer to what the Dark Link user's trainer switches out to, making the move function much more fairly.
However, dropping the +2 Speed boost makes the move strictly worse than Shell Smash, in terms of stat increases.
Dark Link: +4 net stat boost, one turn to transfer stat boosts, user faints, requires only one move
Shell Smash: +6 net stat boost, two turns to transfer stat boosts (with Baton Pass), user doesn't faint, requires two moves (Shell Smash + Baton Pass)
Competitive battling exploits notwithstanding, the user fainting is much more of a penalty than -1 Defense and -1 Sp. Def. Requiring only one move and one turn to transfer the stat boosts, though, arguably balances out that penalty. I typically don't use Shell Smash as a move to compare other moves with, because Shell Smash is an exception to the usual +1 and +2 net stat boost moves. When creating Dark Link, though, I felt that the user fainting could warrant such a drastic increase in stats.
Regardless, my first priorities (no pun intended) with creating moves for this thread are the flavor and in-game balance. But I always try to make the moves I create as balanced for competitive battling as possible.