You're the only one who thinks that a mechanical airframe will outlast an electronic component
Name one instance when an fighter aircraft's radar had to be replaced by a new radar of the same type because the old one 'wore out' !!! Fighter aircraft engines get replaced...new engine of the same type....but radars if any are replaced ONLY for an upgrade.
The radars for Pechora etc (that see 100x operational hours per month than an aircraft) has been operational for nearly 50 years now!!!!!
Irrespective of when the upgrade starts and how long it will take to accomplish it,
it makes no sense to buy new planes in old config! Why can't the new planes being built come in the new config??? Instead of the old radar they'll need to drop in a new one, instead of the old engine they need to drop in a new one!