1) Jagex kills the series. The quest line is locked in its own bubble. In game timeline, the events of the quest happened decades ago, we're just bringing the place back up into operating order. There's nothing that necessarily says they have to keep doing it.
2) Continue as planned. We have 6 runic tiers to go thru. That could take 2-6 quests to complete depending on how they want to do it.
3) EW 4.5 Quest that cuts it short, but gives us pants. Devs tell the rest of the story of the workshop in narrative. The workshop itself can't go on (ie: it is underground, beneath a lake, and not repaired in decades).
4) Scale back some of the puzzles. This would allow them to 'rush' a few runic natures. Afterall, Vitruvius was up there in age and had spent a lot of money on the body machine. So it would make sense that he'd go 'cheap' where he could. Also, Vitruvius was a genius at magic, smithing, engineering, etc. But who is to say he had a green thumb or a knack of summoning. Or that even certain bars were even usable in the way he was used to. We could have like a Nature Bar that Vitruvius found no use for, but a simpler mind (the World Guardian) stumbles across it being a "seed". IT grows weekly/monthly into a tree that a farmer can 'train' on. The puzzle for it was also deceptively simple: a farming plot. Astral Bar could be the same. The only use for it is a focal point for training summoning energies (you focus on it for a bit, it expires, you get Summoning XP). No one said Vitruvius had to have any link to the Summoning Plane (or at best could summon a Dreadfowl Chicken Egg). By cutting back on puzzles, it would allow the devs to do the series in 2 quests instead of more.
The Elemental Workshop 5, while prototype, is the only realistic one to come out. At some point. That's only because Ew5 is an inevitable thing. Just because he's putting together something today, doesn't have any guarantee of a 2020 dev/release.
The Arraxxor quest is just trying to get something together for a concept. No guarantee it'll come out or even make it beyond whatever someone does over this time period.