When I did last house around 1990 getting RCD's in side the consumer unit or fuse box not sure when the type tested type came out, was hard, and the RCBO was only around as a two unit width two pole device.
So two RCD's and two fuses boxes with MCB's replacing the fuses. Today there is no excuse for using RCD's the RCBO's have dropped in price and having just lost a freezer full of food when a RCD tripped simply not economical to fit RCD's feeding many MCB's any more.
But at what point does one upgrade? When you have a serviceable distribution unit at what point does one say this needs changing? And what do you do with the stuff removed which is OK, and as it was when new?