The argument against upgrading

No upgrades.  Where’s the fun in that.  One of the best parts of this hobby after listening to music you love.  

This is already an issue..

Before making the change, I was already able to get into and enjoy certain recordings.

This guy is mostly right.

https://youtu.be/n6hO-y9YU8U?si=T_OLsF0UTVCRS1F4

Further...a single rig may never cut it for a very nitpicky guy...

What’s your view on the benefits of upgrading? How can we reliably assess whether it is effective?

Maybe it helps if you have a specific goal, a target to shoot for.

While many started small then built/upgraded over the years, I started late so my plan was to: 
find out the sonic presentation I that resonates with me (make you sit down, engrossed/lost in the music) then try to create at home within budget.

After years of intense research, I searched and demoed the best speakers at audio shows and stores to find the sound I love which were the Magico and YG with top SS electronics. Unfortunately my budget says otherwise.
At AXPONA I demoed the Magico A5 and Rockport Atria II which sounded subpar due to hotel room setup conditions, I subsequently purchased the great sounding AXPONA demo speakers Vimberg Mino D after my dealer negotiated a deep discount AND I raised/blow my budget big time.  I view the Vimberg as a neutral sounding speaker.

I favor dropping the noise floor to hear better.  

For electronics, I went with SS Constellation Inspiration Pre+Stereo because it was on TAS list. I was going to upgrade eventually since the Inspiration is entry level of 4 levels, so seems like a lot of higher sonic options.

It is a struggle to find the next SS sonic uptick with my budget. Top SS companies Constellation, Soulution, Boulder 2xxx, CH Precision, Dartzeel, Gryphon APEX, etc were out of my budget.  I was targeting a rare used integrates CH Precision, Dartzeel, but they may not come up for sale in NAmer voltage.

Audionet created the STERN preamp and HEISENBERG amp as a project where time and cost is not an issue- so to create the very best possible with no constraints.  Reviews were extremely positive, amongst the top electronics in high-end audio.
Then Audionet took their R&D expertise to reduce the cost into a single box while maintaining Sonics - the Humboldt.

The Humboldt integrated garnered fantastic reviews, indicating reference type Sonics.  I view this as the best SS Sonics that can compare with the very best SS components, but at a much more reasonable price. But new price was still out of reach.

Finding a used Audionet Humboldt was daunting as few are available in my NAmer voltage. Luckily I found one in Canada and just recently received shipment. I’m soooo happy and am optimistic it’ll meet my sonic preference. Maybe someday I’ll purchase an additional amp for variety with different Sonics like GanFet, sliding class A, or punchy like Gryphon. 

Shipping/tariff: I barely knew what I’m doing so I hired/paid a Canadian shipping firm to get it across the border, but I got a letter from US Border Patrol saying I owe them money - I’m calling them tomorrow

It doesn't take much to enjoy music-I've done it listening to AM radio in a car with the windows rolled down. Lots of folks enjoy music with a $50 bluetooth speaker and would think its nuts to spend a grand on audio equipment. So, I don't think the question is whether you like music, or like upgrades, or like equipment. It seems obvious that if you can use whatever resources you choose to apply to this pursuit-a function of capacity and priority- in a way that yields higher quality reproduction of the music, you are likely to enjoy the music more. At least that's how it works for me, otherwise I'd still be listening on the AM radio.

What is judged to be an upgrade may in fact just sound different.  
 

Also, of course,  any higher price tag alone can legitimately be called an “upgrade”.

In the end if one feels better about things than before, it’s an upgrade (for them).

What else even matters?

Food for thought.