There's nothing impossible about HyperLoop. It's just impractical, completely unnecessary and solves no problem
You forgot "expensive". The hyperloop is going to need some massive civil engineering unless it can be built on an almost flat and empty plain. That is because changes in direction (in either the lateral or vertical planes) will need to be very gradual to avoid large g-forces on the passengers. So think in terms of a lot of tunnelling, viaducts, and property demolition (no-one will want to live under this thing). Some people would have us believe that it will cost no more than a large oil pipe line - that's just, er .. Hype.
Stockholm to Helsinki in 30 minutes? OK. But we can already do that. It's called airplanes.
Having said the above, railways (and the Hyperloop is one, albeit with funny rails) have an efficiency advantage over planes for a heavy traffic route. Whether Stockholm-Helsinki is heavily trafficked, and whether the Hyperloop can ever be enginered to carry more than a minority of premium passengers, I do not know.