I've never used anything but soap and water on our tires and also never had cracks form on the sidewalls yet.
That said, this morning getting into my wife's vehicle to go to church some weather checks are showing on the sidewalls of the OEM Goodyear Wranglers on one our Trailblazers, the 2005 model purchased new. It caught my eye in the bright sun as "cracks" has been hot on this forum presently. Not cracks but weather checks as they are hairline and on the surface. Vehicle has less than 70K on the odometer and the tires are a month less than 8 years old now and time to be replaced anyway. Tread wear is also getting close to the wear bars. She doesn't drive that particular vehicle that much and said she wants to keep it as she really likes it. So new tires are coming soon. Not complaining at all as it's just an observation. They'll be replaced with Michelins like all our other vehicles have.
Yes, nearly all tire coating do cause drying and hardening of the tire rubber compound with age. It's the price you pay for having shiny tires! some people want shiny tires and some could care less. Various regions have a lot of hot bright sun and/or more road chemicals etc abundant than other areas. Both play real heck on rubber compounds and so does lower inflation causing extreme tire flexing and heating!