For the teams that miss the PO, the players have from mid April to Labour Day to train. But, they also take time off to heal from surgery plus go on vacation for a mental break for 4-8 weeks. Plus, they need the time to work on their skillset and upgrade a weakness.
Final 4 teams really are hard pressed to do anymore than just get some rest and heal over the summer. Which is why it is so difficult to repeat. The fact that TB and FLA (likely) will have made 3 straight finals each over the past 6 seasons is very impressive. Edm looking like they will go again is also impressive under these circumstances.