Don't kid yourself. The purpose is to spread the cost over as many fiscal periods as possible. If the dates align as you suggest, which as of the present, I'll say is a long-shot (recent history shows us that we can almost certainly count on delays), then it's simply a welcome byproduct.
For a capital improvement project like this, the vast majority of the cost will be amortized over an extended period of time. We're talking 20 years or more.
By way of recent comparison, something like MyMagic+ would have been accounted for differently because so much of that project involved labor--increasing staffing needs to train CMs, train guests, provide tech support, deal with guest issues at the resorts and park gates while the new tech was still being rolled-out, etc. Costs associated with writing new software and installing new door locks can be written-off over the expected life of those investments but a lot of the labor increases likely hit Disney's bottom line immediately.
I agree with Reddog that Disney does have a vested interest in staggering their openings. That's something they've been dealing with for decades.
But also remember they are not able to reap the benefits of these expansions until they open. The idea of any capital investment is to increase profitability. Toy Story Land will increase revenues via higher attendance, higher ticket prices, increased merchandise sales, increased dining revenue and more. If they deliberately delay the opening of this expansion by a year, that's a year's worth of lost revenue.
These factors may not have as dramatic an impact in different circumstances. I doubt Magic Kingdom suffered much while the Fantasyland expansion was under way. But at Hollywood Studios, it would seem Disney has many reasons to open Toy Story Land before Star Wars construction provides another (literal and figurative) obstacle to potential park guests.
Again putting that into context... In the "good old days", they built all of EPCOT in 3 years. Today, it takes them that much time to move Dumbo, construct one 30-second kiddie coaster and a basic Omnimover along with a restaurant and M&G area. Apparently, our collective definitions of "not bad" differ slightly.
Admittedly I'm not a construction expert so perhaps someone in that line of work can comment. I suspect increased involvement of local lawmakers, updated building codes, quality and availability of labor force, technology integration and other factors are playing a role.
EPCOT also wasn't hosting 30,000 guests per day during its initial construction. Separate crews could work on dozens of projects simultaneously without worrying about guests' safety or the quality of their experience.
Elsewhere in the industry, Wizarding World of Harry Potter was just over 3 years from announcement to opening. Phase 2 was 2.5 years from announcement to opening. The Universal Hollywood clone of Wizarding World was confirmed by Universal as far back as 2011 but won't open until 2016.
