Nothing about Namco's development means the game will be finished any sooner than without. Assuming an early release will only lead to disappointment.
It depends how much of the staff at Namco Bandai, Nintendo-owned companies, and other 3rd parties are working on these 2 games (which we won't know for a few years).
Tekken x Street Fighter is "about 10% done" one year and eleven months after its announcement.
It could be a different set of staff at Namco Bandai Games. Namco has thousands of employees, unlike Game Arts, which had maybe 100 at the time of Brawl's development.
This game only recently began development, and expecting a release date sooner than Brawl's is more than naive.
3 years (well, 3 1/2 to 5 years) doesn't sound unreasonable. Anything below that is definitely unreasonable though (IMO). After all, this is 2 smash games, not 1.
it is very common for games to have a 3+ year development in the HD-era.
A lot of that time is spent on graphics technology. I think the days of spending $20 on new graphics technology is over, with the different between graphics being so minimal now, especially with new technology. Now the thing where all the money will go is how much something can run, which hopefully doesn't go out of control like the 7th gen graphics war.
Also, the Brawl developers had a lot of help on graphics-related things. Many outside sources helped make/did make several character models, trophies, stickers, and stages.