Volume
Many letters are sent in batches
A lot of demand letters come from a small number of firms working from automated scans of many sites. That doesn't make them baseless, since the barriers are usually real. It does mean yours probably isn't a targeted investigation of your business.
Resolution
Most settle without a trial
A common outcome is a negotiated settlement that includes a commitment to fix the site within an agreed period. Your lawyer handles that part.
Remediation
Fixing the site doesn't cancel the claim
But a fixed site with dated evidence of the work puts you in a much better position than one that hasn't been touched.
Repeat
It can happen again
The same automated scans keep running, so sites that got one letter sometimes get another. The lasting answer is a site that doesn't fail those scans.