@Evvy If newer tires (no signs of checking) & under cover out of direct sun UV light they last for several years. I've had all my trailers sitting outside in the elements for a few years now they were already X yrs old when I bought these Power Units & they are holding up OK. I wouldn't trust them on a long road trip but travelling to close-by destinations... gas station, neighborhood & on the property hasn't presented any tire problems.
As with any tires they will eventually age & will need to be replaced regardless of looks. I think the rule of thumb around here is most tire places won't remount a tire if its beyond its service life of 8 - 10 yrs. depending on the tire store. Though depending on your local climate, UV + Heat & excessive weight & mileage are the tire killers. However, if you put
parking tire covers on them when left parked on premises, & they are exercised now & then & not left to flat spot they can last for a fairly long time.
Also, the mobility factor is good to have in case of an approaching disaster event... wildfire, hurricane, etc. We are prone to fast moving wildfires living out West during summer drought season. My plan if fire is imminent is to hookup to genset trailers & move those & other ranch equipment into our large sandy horse arena where they won't be exposed to any nearby flames.
Same could be said for those in path of hurricanes with enough notice to bug out in time & get to higher ground or out of projected path. However, having a placed genset on its own foundation with generator shed over it has its own advantages... sound deadening, spare parts & servicing materials on shelfs/toolbox has its appeal as well.
Guess it all depends on one's circumstances & intended usage factors.