The state pledges to and agrees with any person, firm, or corporation, or federal agency subscribing to or acquiring the bonds to be issued under this chapter, that the state will not limit or alter the rights vested in the authority until all bonds at any time issued, together with their interest, are fully met and discharged. The state does further pledge to and agree with the United States, and any other federal agency, that in the event that any federal agency constructs or contributes any funds for the construction, extension, improvement, equipping, furnishing, or enlargement of any project, or any portion of it, the state will not alter or limit this chapter in any manner that would be inconsistent with the due performance of any agreements with the federal agency; and cities and towns continue to have and may exercise all powers granted by this chapter, so long as the powers are necessary or desirable for the carrying out of the purposes of this chapter.