1. The team-wide scoring issues are a logically proven system issue, as it's the only variable in the equation that effects every player on the team. Practically every player with a history of offensive production let alone two generational talents didn't all suffer a case of mass amnesia together forgetting how to score, all at the same time conspicuously on the same team under the same coaching staff. The common denominator are the coaches and more importantly the system, it's the only logical conclusion. There may and probably are other significant negative factors contributing to the offensive drought, but the core problem begins and ends behind the bench.
As was highlighted previously there is an almost exact comparison in recent memory with Ovechkin's outlier years of mediocre production compared to the rest of his career corresponding with the overly-defensive systems implemented in the last year under boudreau and then hunter. The majority assumption at the time was that the "league figured him out" or that he was starting to decline with the stats seeming to agree. But once the team rid themselves of the poor coaching purposefully hindering offensive ability, ovechkin is back to producing at elite levels as if he never stopped despite being years older. Players, even elite ones can only do so much under a system designed, intentionally or not to hinder the advantages of their abilities.
2. That said, In malkin's (and crosby's) case he still deserves some criticism though for such a putrid stretch regardless of the system he's forced to adhere to hindering him so much. As was mentioned above despite coaching undeniably hindering his ability in the past, Ovechkin who they're supposed to rival in talent, could still occasionally create his own scoring changes through will to drive through a defender and manufacture a shot, a scoring chance, anything, system be damned. There's no excuse for that many scoreless games in malkin's case, or going almost two straight games with no attempted shots for crosby, which is imo even more unexcusable. If they're purposefully adhering to johnston's system to that extent results be damned to prove a point then they'd better make sure it's made perfectly clear and soon, before it really is too late.
3. Trading crosby or malkin will never improve the team and there's no realistic argument one could make that will prove that statement wrong. Their high cap hits coupled with their NTC's will ensure the team will never recuperate equivalent value outside of such specific and circumstantial scenarios which would be impossible to predict beforehand that such a trade will never be a net positive for them compared to simply not doing it. There's no logical argument for it outside of purposefully making them worse, or desiring so, hence why such a topic is so popular by outside fanbases every summer.