Raising our seed round was brutal. It took 6 months end to end and was one of the lowest points for me personally. I was trying to scrape together $1M in $50k chunks from any angel who would give us money. We ended up having to lean on our background as founders (MIT engineers, winners of the Battlecode programming competition) to convince the first set of VCs to come in.
Once we started showing traction (0-$1M in ARR in 9 months) we went like hotcakes in our Series A and beyond.
The real test is do customers buy. If you can show that everything will follow. VCs are weak predictors of market success. There's some signal, but they get it wrong almost as often as they get it right. If you close 3-4 paying customers I guarantee you they will change their tune. The incumbency argument is pretty weak IMO, particularly in B2B. Markets are so massive these days it's easy to carve out a large niche. For example, Freshworks went public last week even though Salesforce "dominates" cloud CRM.