Let's talk about SPV... Simplified Payment Verification was originally envisioned by Satoshi as a method of verifying if particular transactions are included in a block without downloading the entire block. It's a way to have relatively high security assurance without...

...actually running a fullnode yourself. You're basically querying random nodes on the network. You can compare against multiple random nodes for greater assurance...

...Anyways, SPV is being deprecated out of Bitcoin Core. I understand why: SPV is not perfect as you're still "trusting" random nodes on the network. It's obviously better to run your own full node, so anything short of that ideal gets deprecated...

...But here is the real world consequence of that decision. SPV wallets like BRD (probably the most popular mobile SPV implementation) have to replace SPV with what they are calling "Fastsync". Essentially, what this means is that your BRD mobile wallet will now...

...connect to the BRD Mothership rather than randomly chosen nodes on the network. For the millions of BRD users, who would have otherwise used SPV, they will now trust a central server for all their Blockchain Truth.

I hope the irony here is not lost on you: The vast majority of Bitcoin users (BRD alone claims 1.8 Million users globally) will likely be interacting with the network through a central 3rd party custodian. Even if we take Luke's (unaudited) figure of 100K fullnodes as truth...

...that means less than 5% of Bitcoin users will be interacting with the network trustlessly, while 95%+ of users will rely on a 3rd party central service. I've had some backchannel conversations with various wallet devs and they are all frustrated by this decision...

...becuz these decisions basically externalize the dev costs to them. They need to re-architect. There's no "fee market" for Bitcoin wallets either. FREE is the expectation. For users: the future is either running a fullnode yourself or trusting a central server. No middleground.