Again Mark has actually designed cards while no one in here has. It's very easy to see how something could be better in hindsight rather than when you are designing a set. Especially when standard rotation changes while you are developing a set.Huh! Learn something every day.
Every time we have Flashback or a Flashback-esque mechanic it's the same routine: people looking at Flashback costs as if they're normal CMCs (and not zero-card-cost bonus effects you get after already getting a whole spell) and deciding that they're bad, probably. Then we spend six months actually playing with the cards and everyone suddenly remembers that Flashback Is Good Actually.
It's well established that Maro knows less about competitive balance than literally any poster in this thread and we're not exactly winning team constructed GPs in here.
Yeah, the issue with general-purpose land destruction is that even not very good LD hoses unoptimized decks super badly, but actually making it a competitive strategy involves giving people enough redundancy that they can consistently lock other decks out of playing altogether. Land lockdown is a tempo strategy so it's much easier to make good without having to be oppressive.