Chewing is not itself important to the development of muscles used in speech. While chewing and speech use the same muscles, they are used in different actions; so, training chewing does not improve speech (specifically, the strength and placement of the articulators). It would be like training running through pushing weights with your legs. There are children who do not chew for years (they do not start chewing within the time period when we expect to see speech emerge and start to develop), as they do not intake their food orally for a variety of reasons. They are not more behind their developmentally-same peers in speech developments and proficiency.
“Soft” foods are already quite popular in the adult population (particularly among the elderly); because of normal aging, an adult’s ability to chew and (safely) swallow diminishes for a variety of reasons including (but not limited to) atrophy of the oral and pharyngeal muscles, decreased reaction time, reduced saliva production, and so forth.
In case someone is wondering, my profession looks after oral and pharyngeal swallowing disorders in North America, alongside speech and language.