If they get left without being dismantled for a while, they will get into a very dangerous state where they are about to fall apart and it is too dangerous to even approach them. Then dismantling them will be much more expensive.
Once the Well itself is plugged, what are the realistic consequences of just letting the things rust into oblivion?
> A secondhand vessel is currently worth about $190 per tonne to a shipbreaking yard in Turkey, a price established by the local market in reclaimed steel. Sail on to China and a different market, and the same metal is worth $210 per tonne. At breakers’ yards in Alang in India, Chittagong in Bangladesh and Gadani in Pakistan, they will pay around $280 per tonne. Meanwhile, at the EU-approved shipbreaking sites, which are bound to conform to continental waste laws, and where vessels are dismantled in closed-off quays or dry docks, rates are less competitive: European yards offer zero dollars per tonne, and, in fact, tend to ask a fee to take a shipowners’ junk.
[0] https://www.theguardian.com/business/2017/may/02/where-oil-r...
Another aspect I read on this topic and remember from the Brent Spar [1] case is that the cavities and tanks of these platforms are sometimes filled with unknown and/or highly toxic undefined stuff. Unless you 'clean' this up it might not be liveable.