Hm. They say 8300 new foster carers are needed.
Clearly, and luckily perhaps, the reduction in numbers of adoptions in the last year has not translated into fewer children being taken into care (nor was it meant to, I believe). So that would account for about 2500 of those additional foster carers needed - children who, in previous years, would have left care by being adopted; are now remaining in care; leaving LAs short of FC places.
Could (some of) these children be adopted? Well that is a complicated question! Just a couple of years ago, LAs obviously thought that they could and should be adopted, and the judges obviously ended up agreeing with them. But not so in the last year - for whatever reasons, fewer children were given adoption plans.
In any case, adoption is not the main result of being in care, and is only in a small minority of cases the preferable outcome. A (temporary?) lack of foster carers should NEVER be a swaying factor in making adoption decisions!
If there are not enough FC placements, then IMO a) new FCs must be recruited (perhaps by improving their 'work conditions'?), and/or b) work must be done to prevent children needing to be in care in the first place, e.g. more support provided to families. That is costly, but surely cheaper in the long term than taking children into care. (Caveat: I do fully understand that in some cases being in care truly is the best outcome, and no amount of support would change that.)