I'd guess there are more people willing to pay a little than maintain the FOSS alternatives themselves. I ran my own mailserver for a couple decades, then found myself at a place where I have more pocket change than free time and started paying someone else to manage the hassle for me. That's where I am here, too. It's worth it for me to pay $10 per month to get Kagi's benefits without having to do it myself.
Apparently they believe there are enough people in that boat to make it a worthwhile business plan.
I'd guess there are more people willing to pay a little than maintain the FOSS alternatives themselves. I ran my own mailserver for a couple decades, then found myself at a place where I have more pocket change than free time and started paying someone else to manage the hassle for me. That's where I am here, too. It's worth it for me to pay $10 per month to get Kagi's benefits without having to do it myself.
Apparently they believe there are enough people in that boat to make it a worthwhile business plan.