Does brand aftercare for used gear make sense?

If you don't, how can you expect them to?

Of course, I don't believe used products deserve customer service

You're missing my point. 

I'm referring to the age old rule in marketing about it being much easier to sell to an existing customer. 

In both these instances the brands wouldn't be in my future consideration set - and the products are very good.

I have had great experiences with small companies.   Better than most big companies to be honest.  Sorry you lost confidence in those guys 

I had a custom amp built, it was the best experience ever.   It didn't come with a manual, it's pretty straight forward.    Input 1 , Input 2 , Gain, Speaker Jacks, Impedance Switch...   Lifetime warranty.  If I text or email the builder with a question he gets right back to me.  He is actually building me a companion preamp for the amp he built me.   It's about as Bespoke of an experience than you are likely to find anywhere.   I had no absolutely no reservations about having him build me another piece.   

My DAC is from a small company with decent support given it's size .   Best sounding digital source I've owned .    5 year warranty.  They are on a roll with a bunch of great products, no doubt they will be around for a while 

I expect both companies to be around for a while, I'm 55 ... I could expire before either one ....

I'll use Zesto as an example.   I owned their Leto preamp .   I bought a demo unit, years out of warranty.  Someone , not me, whacked a tube laterally and damaged the tube socket.  The customer service was exceptional.    They are a small company but they are very responsive and are great people to deal with should you ever need service.  

Sounds good to me.

It's always a good idea to check out resources to ensure their reliability. Look for reviews from other customers who have purchased used gear from the brand. Another good resource is to check out the brand's social media accounts and see how they interact with their customers. Are they responsive to questions and concerns? Do they provide helpful information and support? You can also check their digital channels for infos about the people behind these boutique brands. You can use that info to run some personal checks, like any history of filed complaints against them, etc.

By doing your due diligence and checking out these several digital resources, you can get a better sense of a brand's reputation and whether they're likely to provide the level of support and aftercare you need for your used gear. Good luck with your search!