We would not be able to keep the class of player you allude to, short of top class but much better than the names you mention, content to be sitting on the bench week after week as things have pertained over the last couple of seasons.
I genuinely think we could retain someone but for whatever reason, be it financial constraints or focus on other areas, we've not done so.
Other teams around us of similar stature have usually had many more attacking options (at least of a semi-reasonable calibre) than we have.
Therefore, that's why I believe (only my opinion of course) it has been and still is feasible - even more so now we've got money in our pockets.
Southampton for example, while under under Koeman, had the options of Pelle, Jay Rodriguez and Shane Long. Not better quality yet more depth.
Liverpool had Sturridge, Ings, Origi and Benteke - again depth. West Ham had the likes of Carroll, Sakho and Valencia before going to others.
Could say the same with Stoke. I'm not advocating that they're all amazing players who I'd want, but at least they've got some depth in the area.
For whatever reason however, we've simply not seen the need to have different options up front that you could realistically at least rely on.
Never understood why Martinez binned Naismith off for example and then brought in the unproven Niasse - it simply exacerbated it more.