As others have said, there could be one (stagflation would be a potential alternative, that is, a flat market with inflation making the real returns negative). Nobody knows if and when, though. The markets could never be this low again, or they could be down 80% by the end of June.
Which is why I like the asset allocation advice: since we don’t know what’s in the gears, we should invest our money all at once. As @Cortana writes, it is no different to invest it all now than it would be to have invested it regularly by small amounts in the past and having it in now.
But, if you’re concerned by a potential (probable) future dip, either because your portfolio would loose too much value and you’d not feel comfortable with it (or would need some of that money immediately) or because you’d want to use it to take advantage of it, then a 100% stocks allocation is probably not the right allocation for you. A more defensive portfolio could help to ease your mind for the time you need to be more comfortable and better know your risk tolerance. It’s a personal assessment you have to do but you could start right now with a 40-60% stocks allocation, then see in 6 months if you’re comfortable with it, would want to invest more (beware of FOMO - Fear of Missing Out - stocks often go high big right before a fall) or have found the ride too rocky for your taste already.
And if your purpose is to time a potential future dip, then it would be the time to test your mettle, could you invest the rest of your money right after the fall, before it recovers? If you can, good on you. If you can’t, maybe not trying to time the market is a better way for you.
Keeping money aside to take advantage of dips is generally not advised, because you could “loose” more by not capturing the gains leading to the dip than you “recover” when buying the dip. A 60/40 portfolio is considered a rather solid one and is the one used by the Trinity Study to come up with the 4% SWR (Safe Withdrawal Rate) [edit: scratch that, they have tested different portfolios. Turns out I need to do more research before opening my mouth]. As @nabalzbhf wrote, the usual way to capture dips is by rebalancing. If you feel that you are able to aim for more, that is, that you could actually jump in with your money before the market recovers, then going there with a 60/40 portfolio could give you the opportunity to try your mettle. If it happens you can’t, then maybe it was the correct allocation to begin with.