Whether or not your parents can support themselves has no relevance whatsoever regards to the Parent and Grandparent Visitor Visa. Per
V3.110(d), applicants for such a visa
must be sponsored by the principal applicant's child or grandchild aged 18 years or older who meets the sponsorship requirements set out at
E6.
Prior to the new online application system, the acceptable sponsor would have needed to complete a
Sponsorship Form for Temporary Entry and provide this completed form, with supporting evidence, to the principal applicant who would then submit this with their application to INZ. This remains the process required for paper applications, but I believe that the online way of doing things may be different nowadays.