I wouldn't know of the future plans for Mesmers, but a moment of damage immunity is integral to all professions in GW2. Locking the Shatter attribute is opposite to the Mesmer's core functionality, but at least the above would not leave the specialization remarkably dysfunctional in comparison to its others.
If there is a concern for bloat (which in game I have not heard) this would limit the fifth Shatter as conditional. The biggest thing about a conditional Continuum Split is that it is absolutely unreliable to use over the created Rift plus Clone requirement. Breaking either one mid cast essentially doubles the cooldowns of any Chronomancer build.