Your CTO says the architecture needs to be rebuilt
Your CTO says the architecture needs to be rebuilt. You cannot evaluate the architecture. You can evaluate the claim.
Start with what the rebuild is supposed to fix, in business terms. "It's slow" means what, for whom, costing what? "We can't add features" means which feature, requested by which customer, blocked for how long? A rebuild that cannot name the feature it unblocks is being sold on discomfort.
Then ask for the middle option. Every rebuild has one: fix the two worst parts, leave the rest, buy a year. Engineers often skip it because the middle option is unglamorous and does not come with a clean slate. If the CTO has not priced it, ask them to. If they price it dismissively, ask why.
Ask what the business stops getting during the rebuild. Most rebuilds freeze new features for six to eighteen months. That is a cost, and it belongs in the comparison next to the licence fees and the engineering time.
Ask who else has seen the code. One senior engineer from outside, two days, a written opinion. That is a small price for a decision this size, and a CTO who is confident should welcome it.
None of this doubts the CTO. Rebuilds are sometimes the only honest answer. The point is that "rebuild" is a proposal like any other, and it should have to make its case the same way a vendor's would.