Those in support of closing the 710 Freeway gap must not travel the 210 very often because this freeway has its share of daily bumper-to-bumper traffic, particularly during weekday mornings eastbound. This is without the heavy truck traffic that would instantly materialize with the gap's closure. Also, the 5 Freeway is a heavily industrial corridor that favors heavy truck traffic; the La Crescenta, La Cañada and Pasadena environs are decidedly not.
Any cursory view of a map will show that the triangle created by the 5, 210, and 710 freeways gives the shortest, most direct route to and from Long Beach to the 5. Closing the gap simply adds more miles and time on the road for trucks whose cost efficiency would be negatively affected; and, we all know who pays for shipping costs.
The 210 Freeway was unsuccessfully fought tooth-and-nail by the residents and neighbors of the hundreds of homes destroyed by its construction in the mid-1970s. The Crescenta Valley is a unique part of the L.A. Basin (in spite of the 210) and should remain that way. If truck traffic on the 5 is the source of consternation on this issue, my suggestion to people is to advocate for restrictions on heavy truck traffic on the 5. Other than that, keep your hands off the 710 gap.