You're correct. However, I'm fairly sure OP was talking about Segwit (with 2x).
If most users/exchanges/services don't get on board by the time the scheduled segwit2x hardfork happens, then those blocks would be considered invalid.
That makes sense. However from what I've read, the timeline is "SegWit -> multi-month transition period -> 2x HF".
So by the time the 2x fork comes up, the segwit-enabled chain will already be in use for several months.
Another point I'm wondering (sorry if that's a noob question) : Assume that some of the forks actually lead to a chain split with hashpower distributed 90%/10%.
At some point, the difficulty of the chains will adjust to bring both of them back to the "one block every 10 minutes" rate. However, this will not happen immediately but only when the current difficulty period would complete. Before that happens, the minority chain will still have the difficulty from before the split - the one that assumes 10x as much hash power as now is actually available. That sounds as if the minority chain could become effectively unminable for the rest of the difficulty period. Worse, as the period durations are measured in mined blocks, if blocks are mined slower, the difficulty period will also go on for a longer time.
> So by the time the 2x fork comes up, the segwit-enabled chain will already be in use for several months.
Correct. But there's an asterisk here: most people (78%) are running bitcoin core, some run Core/UASF (5%), and some run bitcore (2%). Together, that's 85% of nodes. Core 0.14, Core/UASF, and bitcore are all compatible with segwit, but not with the blocksize increase. Even if you wait multiple months, if people stick with Core (a possible outcome) then they will see 2x blocks as invalid.
> That sounds as if the minority chain could become effectively unminable for the rest of the difficulty period.
The chain wouldn't be unmineable, it would just get mined slower (mining profitability wouldn't be impacted for miners staying on the chain provided the price stays the same). But you're right, blocks will come slower, throughput will be lower, and the adjustment will take longer.
You're correct. However, I'm fairly sure OP was talking about Segwit (with 2x).
If most users/exchanges/services don't get on board by the time the scheduled segwit2x hardfork happens, then those blocks would be considered invalid.