Lol, first of all Mark Stone will age just fine because his game does not involve speed at all. Dude is in his prime right now.
Second of all, we increase our chances at a Cup dramatically (more so than with Knies) because Stone would be our best winger. You can put him with Suzuki or Caufield and slot Slaf with Demidov. At that point, your 2C wouldn't have to be Hischier, you could get away with an improved Kapanen.
If we win the Cup because of it, nobody will care we gave up Zharovsky. The 1st rounders are late ones, they are overrated in value.
*This is in the hypothetical scenario that Stone is never injured.