He still has his original tank cannon form and it doesn't look compromised by his space cruiser mode. I don't see the issue with offering him with an alternate transformation scheme that makes more sense for transport (something I always considered a central feature of alt modes). It's not like this an MP. Artistic license is totally expected in this line.
I don't have a problem with them tossing out a new transformation, I just don't think he needs it.
I don't think movement is the central feature of transformations. I think it just feels that way sometimes because of the way Autobots behave when they're fighting on the front lines. They (mostly) transform into vehicles for bursts of speed, and then transform into robots to have better access to their guns. But not everybody does that. Perceptor, for example, is locked into his robot mode in a fight, because his microscope form is more suited for lab work and is useless in a fight. Blaster is similar to Soundwave. The Dinobots don't exactly get a mobility boost by transforming.
Most of the Decepticons can do the exact same things in both robot and jet mode (fly and shoot, in either mode), but they seem to transform into jets for a slight increase in their air superiority advantage over the Autobots, and transform almost-exclusively into robots while on the ground (because grounded planes are useless). Megatron and Soundwave fight side-by-side with the jets (on the ground and in the air), and never seem to miss a beat.
Megatron's transformation was never about mobility. He becomes completely useless when he transforms, and needs to have another character wield him (I've speculated that he was built to be a tool of war, under somebody's tight leash, but he cast that off long ago). Galvatron has some increased degree of mobility (he can aim
himself, thankyouverymuch), but he's mostly just a stationary "big gun". If he fights Starscream in that form, it's overkill. If he fights Unicron, it's not enough. Galvatron's alt-mode was seemingly built to fight Combiners, and there's nothing wrong with that, even though most of the time he's just fighting regular-sized bots and his robot mode is more suitable, so he rarely transforms.