I'm not so sure it's a great time at all. Most alzheimer's drugs have failed dramatically. vTv Therapeutics is the most recent I think. They had a Phase 3 drug trial end early due to futility, and their stock tanked hard. In one day I think it lost 70% of its value.
As far as I'm aware the majority of failing biotech companies have very broadly targeted Amyloid beta (apparently because they thought it was a good risk tradeoff). A narrow focus on APOE4 could be a wiser strategy. I could even see gene therapy playing a role.
There are a few well funded startups targeting apoe4 and other genetic markers of neurodegen disease. Exciting field
Alzheimer's drugs have not been successful, but in general it is the best time ever to start a biotech company. Returns in biotech VC the last 5 years are better than tech, more IPO and big M&A (three $8-12B startup acq in last 4-6 months) even with 1/5 the funding of software, tons of great science out there, massive amounts of capital ($4B venture funding to startups in Q1)