Well in such a situation a block would never be created with an invalid transaction because miners are (generally) going to be running fully validating nodes. As such, SPV clients would at worst only see it as an /unconfirmed/ transaction. Though a slightly similar situation occurred a couple years ago with the BIP66 soft fork where some miners were NOT running full nodes and they created an invalid fork that was 3 or 4 blocks long. But they realized pretty quickly that they were never going to be able to claim the block rewards from that chain and they switch backed to the valid chain out of economic interest. With regard to SPV I have a few articles that are relevant. One on the security model of full nodes: https://www.coindesk.com/bitcoins-security-model-deep-dive/ And one on the scalability issues inherent to SPV: https://www.coindesk.com/spv-support-billion-bitcoin-users-sizing-scaling-claim/

It's basically impossible to quantify "safely decentralized" though we seem to be doing fine lately - the ecosystem at large has become much more efficient in its use of block space. We aren't maxing out block weight by any means yet and great strides are occurring with Lightning Network, so I think we're in a pretty good position. I doubt we will ever have a significant portion of bitcoin holders that verify everything, but then again everyone has the freedom to choose their own security model. So long as it's possible for users who WANT to have the strongest security model to afford the hardware required to run a full node, I think we're in a healthy state.

One of my main issues with proof of stake is the nobility problem - the rich get richer without having to expend significant resources. I prefer proof of work because it's more dynamic - new participants can enter the network without having to "buy out" existing participants. It seems more like a competitive free market.

While some nations such as China and India have cracked down a lot on crypto, others like Japan have become much more risk averse and want to protect users, while some such as the US seem to be generally embracing it. From what I hear, a number of politicians already own bitcoin, so hopefully that number continues to rise to disincentivize them from legislating such counterrevolutionary moves... but I do think that the most devious thing nation states could be doing is working to pit us against each other in order to distract us and slow down the pace of innovation.

Some ups, some downs, and some sideways. There will be no shortage of drama and fud as folks jockey for relevance.

It's an apples and oranges type of comparision, but in general open systems are going to be more secure because they have more eyes on them, probing for vulnerabilities. Note that banks get hacked all of the time... At a very high level, money you keep in the bank for the next ten years is GUARANTEED to be worth less than when you deposited it. Bitcoin offers no guarantees either way, but it has a huge upside potential.

It's all interrelated - sometimes stuff I tweet comes from work conversations while some of the stuff I find on twitter helps me suggest new things for us to implement at work!

DAPP is a very broad term so it really depends upon the functionality you want. But in general it's going to require learning how to interface with a new network such as Bitcoin / Lightning / Ethereum. There are Lightning resources at https://dev.lightning.community/ For Ethereum you'd likely end up writing a smart contract so you'd need to learn Solidity - take care that it's easy to write a "working" solidity app that is insecure!

If a critical protocol bug was exploited and not found for a long time after being exploited (weeks or more) then it would cause a huge loss of confidence. I still believe that the most likely cause of Bitcoin's death would be apathy. Everyone has to agree to stop supporting the project in order for it to die.

At retailers? Not any time soon, though we will ship to Europe! Yes, sour beer please.