I didn't mention them playing in the top 4 next year, either. But that doesn't change my point. You don't clear out space for them, even 2-3 years down the line. If they push their way onto the roster, you deal with that roster jam at that point. It's a good problem to have. You don't walk away from Kempny because you have a handful of guys playing in Hershey who might be ready in 12 to 36 months.
Prospects are called prospects for a reason. There's no such thing as a surefire prospect, and the ones that the Capitals have aren't even tier 1 prospects. The Capitals are competing now, not in a rebuilding phase. The prospects will and should have to earn their spots on the roster, just like the young kids did on this year's team. Remember that the Capitals went out and grabbed DSP this past off-season and invited Chiasson and Jokipakka to training camp. Taylor Chorney was still around, as well. Djoos and Bowey had a relatively easy route to roster spots this year, but that was mostly due to a cap crunch and the expansion draft opening up spots that the Capitals couldn't afford to fill. And even then they still needed to beat out Chorney and Jokipakka for jobs. And ultimately the team was better later on, once Kempny and Jerabek were brought in (pushing Bowey and Chorney out of the picture and giving Orpik and Djoos more comfortable roles). The task was much tougher for forwards, and Chandler Stephenson started the season in Hershey. Chiasson beat out a number of the Hershey kids for a roster spot. DSP stayed in Washington despite signing a 2-way deal. Nathan Walker ended up losing his roster spot later in the season.
You can find similar stories on the Penguins teams that won the Cup the past two years. Guys like Guentzel, Sheary, and Rust had to outplay others to earn their roster spots.
This has nothing to do with "optimism" about the prospects, and everything to do with not passing up a good opportunity to lock up a 27 year-old Top 4 D at a reasonable price.