False praise does not help
"The era of a phone that can also be a tablet or a full computer is arriving, thanks to FOSS."
The era of a phone that can also be a tablet or a full computer already got here. Android's had support for that for a while with a few different companies' standards alongside the half-baked Google version for every device (some exceptions apply because it's Android, exceptions always apply). People don't use it.
Yes, Linux is a better desktop operating system than Android is, so a Linux phone should be better at it, and it probably will be when we have Linux phones better than this. This phone is not capable of running a desktop system very well based on its specs. From reviews, it's not that great at being a phone either. You've complimented the CPU (incorrectly, but that's for later), but there are other problems as well. Here's a simple one: 32 GB of EMMC storage. That's what you have to put your desktop operating system on: a disk that's smaller, slower, and less reliable than anything you would ordinarily use. The good news is that you can put in a Micro SD card for some expansion, but it's not going to be fast. 3 GB of RAM isn't exactly impressive by 2013 standards either, but that is less likely to be immediately problematic if you don't open too many browser tabs.
Now onto the CPU. You've told us that "it's easy to forget that even a relatively low-end smartphone is still a powerful computer by the standards of just a decade ago", but this is incorrect. Four A53s are not fast when compared to a 2013-era computer processor. I don't have good benchmark numbers for the specific SoC used in the Librem 5, but those are standard cores where performance is proportional to clock rate and memory speed (that appears to be the same), so let's look at another quad-A53 chip, specifically the Qualcomm Snapdragon 410 (MSM8916). It got Passmark benchmark numbers of 424 at 1.25 GHz, so for this devices maximum 1.5 GHz (not specified how often it will operate at maximum), we'll adjust its number to 509. Let's look at some CPUs from 2012 (we're not that far into 2023 anyway) and see what their numbers are like. I don't want to use anything too powerful, so I'm looking at small laptop CPUs. How about the Core I5-3317U, a dual-core 1.7 GHz part that's far from the top of the rankings. What's its benchmark number? 1988, 291% above our adjusted Librem number. AMD hadn't ascended to its present position at that point. Let's try one of their parts. How about the low-end APU A6-3650: 2008. But that one has four cores. How about a dual-core model like the Athlon II X2 240e? This is by far the worst I found and it's number is ... 1002. Some phone processors are powerful enough to serve as laptop-class chips, but this is not one of them. You don't by a Librem 5 for the CPU. You buy it because you want open software and are willing to compromise on basically everything else to get it. People know that and it's fine. Don't pretend it's something it isn't.