I know it's not for everyone, but it is also a reasonable touring bike if done within its constraints. I've probably done closer to 40 days on my 16" Brompton, longest was a two-week 1000km ride. On the topic of leaving the bike out-of-sight: In those 40 days I've left it locked a total of 20 minutes, otherwise it comes with me into restaurants, supermarkets, public restrooms, hotel rooms..

The biggest downsides are speed and climbing ability. 80k or so has been a reasonable max distance on tour (I've done one 100k day, it was long) and I wouldn't take it to the Alps.

Like the OP, I run Schwalbe Marathon Plus which has been good. But I have had one catastrophic puncture after riding over a particularly nasty piece of glass that cut straight through the tire. After that I bring a folding backup tire.