This is not a question about the meaning of "could" and "would". This is a question about which word fits the context.
I got conflicting remarks on my paraphrase of a few sentences. I am not sure which is more valid.
Here is the original text (Note that the issue is about a bill that was introduced in 1971):
Regionally extended benefits were introduced on the grounds that the additional costs related to the presence of structural unemployment in some regions should be shared with other regions and taken on by the government ...
This is my paraphrase (I used "could"):
... regionally extended benefits, which were introduced so that the additional costs of structural unemployment in some regions could be subsidized by government contributions and EI revenue from other regions.
Reader A's feedback (emphasis mine):
I think it is a reasonable paraphrase. However, the wording of the original is rather circumspect. It expresses the reason for introducing the benefits more as a hope or aspiration than as an expectation, and it does not say what the actual effect was. Your use of "could" might be too assertive, and "might" might be better.
Reader B's feedback:
Looked at another way, 'could' could be seen as too tentative! 'Could' could imply that this was possible but might not happen (or that a choice was available). The original sentence tells us that the argument in favour of regionally extended benefits was that the additional cost in one region should (for ethical or practical reasons, or both) be shared with other regions and undertaken by the government'; regionally extended benefits were introduced for this reason; it is an inevitable assumption that the purpose of legislation based on a given argument is to make compulsory the action proposed by that argument . To reflect this,use "would":
... which were introduced so that the additional costs of structural unemployment in some regions would be subsidized by government contributions and EI revenue from other regions.
This is not a generalization about the possibility of replacing reason clauses with purpose clauses, and which verb (could,should,might etc.) is appropriate, which in other contexts may involve no ambiguity; but here the context does make a difference; legislation may be permissive ('You may do this') or imperative (You must do this'), and the conventional use in purpose clauses of 'could' or 'might' can, as here, be ambiguous.
I wholeheartedly agree with Reader B's feedback, except on the use of "would". It just doesn't sound right to me. So, I guess my question is, which makes more sense based on my context?