if i had to resort to using one bike (especially if it's a steel tourer..) i'd probably give up cycling altogether
This.
I got a Domane Disc 4.5. It's an amazing bike, and feels so reassuring. The big tyres feel like they have a multiplied effect from the suspension. I imagine the SLR feels like a fatbike, albeit an 8kg fatbike with drops. Additionally, thru axles - they make a bike feel so laterally stiff, you feel like you can carve corners at insane speeds. I am current 4th out of several thousand going down Norwood edge, despite the fact the tarmac is currently screwed, and it was just me messing about on a commute. It's like comparing an enduro bike with a hardtail in terms of descending safely. The discs are immense. 160mm rotors with hydraulic calipers - there is always more stopping power than you need - until you hit gravel then all bets are off.
Is it one bike to rule them all? Not at all. If I want to kick to keep up with quick mates on a club rides, I take my jittery, hectic 7kg Madone with light wheels, a 120g carbon saddle and bars and a seatpost so stiff, you feel everything. However, I could take the domane on gravel rides, and for audaxing it is a beast. However, I wouldn't go bikepacking offroad on it like people do on Shand Stoaters or that Ti Kinesis that takes 40mm tyres.
If there was one bike to rule them all, this might be a bit closer.