Except the timing doesn't really work. If you wait a few years to sign a free agent to augment the core, you are going to be doing so around the same time that Hughes, etc. would be needing their next big contract.
If you get Hughes or Kakko, you sign that elite free agent NOW, for several reasons:
(1) they are available this year, which is not very common;
(2) the earlier you sign them, the less % of the cap they will take up, and term they will have left, by the time Hughes/Kakko needs to get paid (i.e. next year's cap is roughly $83 million. If you sign Panarin now to a $10/mil per deal, he would take up 12% of the cap. In 4 years, assuming the cap goes up $3 million a year, he would only be taking up 10.5% of the cap, would continue to trend down, and would only be signed for 3 more years by the time you need to renew Hughes/Kakko);
(3) Lundqvist, Staal, Shattenkirk, Smith all come off the books in 2 years. That alone frees up $25.2 million, giving you the flexibility to sign an elite player (which I consider Panarin and Stone to be, ages 27 and 26 and with games that are unlikely to suffer a steep decline), now, while they are available.
(4) Yes we need to continue building through the draft, but we have made 5 first round selections in the last 2 years, not to mention this coming draft, not to mention Howden, Hajek, Shestyorkin, Rykov. The pipeline is already getting deeper, we are just missing an elite talent or two. Drafting is one way to get them, free agency is another. We have the cap space and resources to do both.
(5) There is only ONE elite free agent possibly available in the summer of 2020 - Taylor Hall, who you would be asking to cross the river. I'd much rather pay out guys like Panarin and Stone who can produce and generally stay healthy as opposed to a guy who is 1 year older and always misses games.