All teams are holding back on developments. This is F1, you don't come up with something and bolt in on the car the next day, you have lead teams for production, analysis to work out whether it's worth extra dev time for resources vs reward against other parts and avenues.
Development is a continous process down many avenues in parallel - why do you think they all wait as late as possible to build the cars up before testing?
Why do you think every team always says "we have new packages coming for race day" in testing?
By your reasoning they should have just bolted them on now.