Tbh my main issue wasn't the cancellation in and of itself but the way it was handled/announced, there are so many better ways it could have been worded.
If you need to take a break for whatever reason, whether it's mental health or medical, then take that break but you have to acknowledge that some people are going to be left out of pocket due to things totally out of their control when things get cancelled and they're more likely to be supportive if they feel that the artist is only doing so because they have to, not just that they didn't consider the pressures of touring/performing before embarking on a solo tour and taking their money. Even if the cancellation due to needing to "just be Lewis from Glasgow" is a smokescreen for something more serious, it is undermined by the fact the cancellation announcement is all about needing a break in order to be well in time for Glastonbury, as opposed to him wanting to take a break and then when he is ready to return deciding when he's going to do that first performance.
The cancellation essentially comes across as "I'm rescheduling everything because I'm burnt out and want to focus on being able to perform at Glastonbury. I know some of you will be left out of pocket but my need to be ready to perform at Glastonbury is my priority" Which he is perfectly entitled to do, but when you're cancelling weeks of shows where hundreds of thousands of your fans are coming to explicitly support you because they love you and your work so that you can prioritize a festival where you aren't even headlining and at least half the festival goers will be elsewhere during your set it feels very dismissive of those fans.
I would have thought it better to have made an announcement saying he was cancelling the rest of his tour dates as he was suffering with his MH and he needed a break and would decide on any other commitments eg festivals etc at a later date but would keep people updated as much as possible. Then at a later date (even if it was just after the weekend) he could have put out a separate announcement saying "thank you for all the love and support I've received since the announcement and for the understanding of my cancelled shows. It wasn't done lightly and I appreciate the support. The break from performing has been good for me and I'm already feeling better so my aim is now to be ready to return to perform at Glastonbury in a few weeks"
I am not a Capaldi fan, but one of my friends is and we had tickets to one of the now cancelled shows ... She has been looking forward to the show for months, we are both in low paid jobs (she works in hospitality and I work in retail) so the cost of tickets plus travel to the city and accommodation was a not insignificant sum to us, about £350 each, and that's before I include meals/drinks etc.
We were both happy to pay that much money to treat ourselves as a one off birthday weekend luxury - a child free adult weekend where we could get the train, check into a nice hotel (again since it was our birthday treat we went for a nicer hotel than just a budget Travelodge), go for dinner, see the show, stay over and have a day to go shopping/sit and have coffee and gossip etc before getting the train home to the regular 9-5 daily grind.
Now we are left with either losing money on the trains/hotel or going to a city neither of us are particularly bothered about visiting (it's one of the nearest big cities to us but is about 2hrs away so whilst we don't pop in every weekend, we visit regularly enough that it isn't a "destination" to visit in the same way a city further away might be) and either trying to book something else while we are there (costing more money that hasn't been budgeted for at short notice) or just spend the weekend in the hotel so we can feel like we've at least not wasted the money ... Then if the dates get rescheduled then we have to decide whether we want to spend more money that we don't have to attend that gig and risk it happening again.
My friend has already messaged me to ask if she should just refund the tickets because the announcement left a sour taste in her mouth and she doesn't want to see his gig any more - she understood and totally sympathised with him needing a break but the fact he thought fans would be cool with him prioritising a festival over his solo shows just meant she felt like he was treating them as second class and she doesn't want to spend money seeing that artist any more, she'd rather spend it elsewhere.