Obsoleting Wonders is not the same with replacing Wonders. The variant didn't say that you can't replace Wonders with another.
Indeed replacing is different -> The question only remains on own obsoleting with Monarchy or Gunpowder.
The utility of such moves are very rare. The majority of this use is to save another player by weakening yourself. For example, a 3 players game.
Egypt have Great wall, Civ A is agressing Civ B who is quite weak and will be an easy target for Civ A. Civ B is the first player to move this turn and will be attacked by Civ A. Civ B needs an Iron to win the battle (Civ B have Math) but have no acces to it. -> Egypt harvest an Iron during CM, obsolete their Great wall and let Civ B attack one of his amry to loot the needed Iron.
Same for trades, if ever Civ A play disoriented during CM and bring some Civ B armies in range of an attack. -> Egpyt and Civ B were surprised and Egypt didn't give some trades to Civ B during trade phase. -> Egypt obsolete their Great wall to allow a lot of 3 trades from Civ B. -> Civ B can research his lvl 3 tech that will save his ass as Civ A would have looted 3 trades to forbid the research of the Lvl 3 tech.
Of course, these cases have like 1% chance to happen in a game but it can still happen ^^.
There are also some examples with Brandebourg Gate, Statue of Zeus (quite the same example for this wonder to allow the victory of Civ B vs yourself).