This thread has over 17 pages (plus more on the other thread) consisting to a large extent of people pointing out the obvious absurdity of compulsory reservation in most situations (and some raising the possibility of associated changes which would further reduce the flexibility and/or affordability of rail travel), and a smaller number of people who fail to see that absurdity; but we don't actually know a) what exactly is planned, b) how likely it is to happen, and c) what to do about it.
On a), “compulsory reservations will be a part of LNER’s future” could mean universal compulsion (with all the disadvantages that have been described upthread), or apply only to departures from King's Cross at a few busy times (which would probably have a net detrimental effect, but wouldn't be entirely pointless), or something in between (I wouldn't be entirely surprised if we ended up with a system where reservations were theoretically compulsory for all travel on LNER but this was only enforced at particularly busy times).
On b), it could be that LNER have been given a firm instruction by the government, or that Mr. Horne was just flying a kite on his own account to see what the reaction was, or again something in between.
On c), we can argue all we like here, but decisions about these things are made by the senior people in the DfT, LNER and the other TOCs. Does anyone have any suggestions as to how best to influence their decisions in a passenger-friendly direction?