[zelphirkalt]

It does look nice, and covers most things, except for life lines. Is there a way to add life lines?

And why do so many tools choose to ignore life lines and provide incomplete sequence diagrams? Perhaps it gets really complicated to express in such a simple way as the examples do?

[quietbritishjim]

It has those! They're under "Lifeline Activation and Destruction" on that page:

https://plantuml.com/sequence-diagram#5cc0040514e70f7b

You can even have two active on the same lane at the same time, as shown in the second example on that page.