It might be useful to consider what type of magical capabilities a dragon has in 3.5E, before we argue one way or the other what the loss of spellcasting will do.
An adult red dragon is CR 15 and can cast spells as a 7th level sorcerer; it can also cast spells from the cleric list, as well as spells from the Chaos, Evil, and Fire domains. Functionally, this means that the dragon can cast spells up the 3rd level (not very high). In addition, the red dragon only knows 5 1st lvl spells, 3 2nd lvl spells, and 2 3rd lvl spells. A very cursory examination of the PHB spell lists brings up these spells, which could provide vital non-combat capabilities to a dragon serving as a major antagonist:
Lair building: magic mouth, stone shape
Traps/wards: alarm, arcane lock, explosive runes, glyph of warding, hold portal, misdirection, nondetection, Nystul's magic aura, obscure object, sepia snake sigil
Information gathering: augury, clairaudience/clairvoyance
Allies/minions/slaves: charm person, status, suggestion
Note that other spells could be used to create magical device traps, but I have not listed these for the sake of brevity.
I think many of these capabilities could be accomplished with magic items or magical traps (crafted using a scroll of the appropriate spell), or perhaps a supernatural ability like "lair sense" or "hoard sense" that grants alarm spell-like powers to the dragon when it is in its lair.
The loss of spellcasting becomes a much bigger deal when you start talking about CR 20+ dragons that have caster levels in the teens and above.